DHS METHODOLOGICAL 

REPORTS 14

Potential Bias and Selectivity 

in Analyses of Children Born 

in the Past Five Years Using 

DHS Data

SEPTEMBER 2014

This publication was produced for review by the United States Agency for International Development. It was prepared by 

Shea Rutstein of ICF International.

DHS Methodological Reports No. 14

Potential Bias and Selectivity in Analyses of Children 

Born in the Past Five Years Using DHS Data

Shea O. Rutstein

ICF International 

Rockville, Maryland USA

September 2014

Corresponding author: Shea O. Rutstein, International Health and Development, ICF International, 530 

Gaither Road, Suite 500, Rockville, MD 20850; Phone: 301-572-0950; Fax: 301-407-6501; Email: 

Shea.Rutstein@icfi.com
Editor: Bryant Robey 

Document Production: Yuan Cheng 

This study was carried out with support provided by the United States Agency for International 

Development (USAID) through the MEASURE DHS project (#GPO-C-00-08-000080-00). The views 

expressed are those of the author and do not necessarily reflect the views of USAID or the United States 

Government. 

The DHS Program assists countries worldwide in the collection and use of data to monitor and evaluate 

population, health, and nutrition programs. For additional information about The DHS Program contact 

DHS Program, ICF International, 530 Gaither Road, Suite 500, Rockville, MD 20850, USA; phone: 

301-407-6500, fax: 301-407-6501, email: reports@dhsprogram.com, Internet: www.dhsprogram.com. 

Recommended citation: 

Rutstein, Shea O. 2014. Potential Bias and Selectivity in Analyses of Children Born in the Past Five Years 

Using DHS Data. DHS Methodological Reports No. 14. Rockville, Maryland, USA: ICF International.
iii 

Contents 

Contents ...................................................................................................................................................... iii

Tables ........................................................................................................................................................... v

Figures .......................................................................................................................................................... v

Preface ........................................................................................................................................................ vii

Abstract ....................................................................................................................................................... ix

Executive Summary ................................................................................................................................... xi

1. Introduction and Purpose .................................................................................................................... 1

2. Characteristics of Most-Recently-Born Children and Non-Most-Recently-Born Children 

Compared with All Children Born in the Most Recent Five-Year Period ....................................... 3

3. Biases in the Effects of Subsequent Inter-Pregnancy Intervals on Nutritional Status and 

Mortality under Age 5 .......................................................................................................................... 7

3.1 Nutritional Status of Children under Age 5 .............................................................................. 7

3.2 Under-5 Mortality by Succeeding Birth-to-Conception Interval .......................................... 10

4. Conclusions and Further Research Needed ..................................................................................... 13

References .................................................................................................................................................. 15

Appendix .................................................................................................................................................... 17

v 

Tables 

Table 1. Children born in the five years preceding the survey living at the time of the survey, 

45 DHS surveys .............................................................................................................................. 5

Table 2. Birth interval by age of child .......................................................................................................... 7

Table 3. Percentage of children last born and with succeeding birth-to-birth intervals, mean 

succeeding interval length, height-for-age standard deviation, and percentage stunted by 

child’s age, for living children born in the five years preceding the survey, 40 DHS surveys 

(255,760 children) ........................................................................................................................... 9

Table 4. Excluded deaths by length of succeeding interval, children born 1 to 15 years prior to 

survey, 52 DHS surveys 2006-2012 .............................................................................................. 12

Appendix Table A1. Types of DHS data available on children, by restriction ........................................... 17

Figures

Figure 1. Mean height-for-age z-score (WHO), by age of child .................................................................. 8 

Figure 2. Proportion stunted, by age of child ............................................................................................... 8 

Figure 3. Child mortality by length of succeeding birth interval, children born 1 to 15 years prior to 

survey, 52 DHS surveys 2006-2012 ........................................................................................... 11

vii 

Preface 

The Demographic and Health Surveys (DHS) Program is one of the principal sources of international data 

on fertility, family planning, maternal and child health, nutrition, mortality, environmental health, 

HIV/AIDS, malaria, and provision of health services. 

One of the objectives of The DHS Program is to continually assess and improve the methodology and 

procedures used to carry out national-level surveys as well as to offer additional tools for analysis. 

Improvements in methods used will enhance the accuracy and depth of information collected by The DHS 

Program and relied on by policymakers and program managers in low- and middle-income countries. 

While data quality is a main topic of the DHS Methodological Reports series, the reports also examine 

issues of sampling, questionnaire comparability, survey procedures, and methodological approaches. The 

topics explored in this series are selected by The DHS Program in consultation with the U.S. Agency for 

International Development. 

It is hoped that the DHS Methodological Reports will be useful to researchers, policymakers, and survey 

specialists, particularly those engaged in work in low- and middle-income countries, and will be used to 

enhance the quality and analysis of survey data.

Sunita Kishor 

Director, The DHS Program 

ix 

Abstract

This paper investigates potential biases in analysis of data for only last-born children or non-last-born 

children instead of all children, and in the analysis of succeeding birth or pregnancy intervals, in both the 

five-year period preceding the Demographic and Health Surveys (DHS) surveys and for longer periods of 

time. Beyond the usual considerations of omission of births and transference of births across 

questionnaire age boundaries, using data for only last-born children or non-last-born children instead of 

for all children born in the five years preceding the survey can result in biased research findings. 

Unfortunately, for certain child health outcomes, some DHS data are only collected for last-born children, 

and other data only for non-last-born. The correction of one bias may create another bias, as in the 

estimation of mortality risks by succeeding interval. Given the likely negative effects on the health and 

well-being of children whose birth is followed quickly by another birth, substantial efforts should be 

made to try to overcome these biases. 

xi 

Executive Summary

Many studies have shown the deleterious relationships between the length of preceding inter-birth or 

inter-pregnancy intervals and the pregnancy, birth, and child following the interval. The conception and 

birth of another child after a short period of time would also be expected to negatively affect the earlierborn child. These considerations have led researchers to attempt to use data from The Demographic and 

Health Surveys (DHS) Program to investigate health outcomes linked to the subsequent birth or 

pregnancy interval. Some of the outcomes of interest have data collected only for the five years preceding 

the survey (e.g. current nutritional status and vaccination rates), while others can use earlier data (for 

example, child mortality). However, the nature of the problem and the structure of DHS data can lead to 

serious biases in such analyses. It is the purpose of this Methodological Report to describe and illustrate 

some of these biases. 

In 45 recent DHS surveys, of the nearly half a million children born in the five years preceding the 

survey, over 70 percent are the most recently born (last birth). The demographic and social characteristics 

of last-born children differ from those of all children born in the five-year year period preceding the 

survey. Last-born children are slightly less likely than non-last-born children to be of low birth order. 

Forty-seven percent of last-born children are of birth orders 1 or 2 compared with 51 percent of non-lastborn children. Last-born children are more likely to be male than the non-last-born, 51.6 versus 50.0 

percent, respectively. Last-born living children tend to be younger than non-last-born children: about 24 

percent of last-born children are age 3-4 compared with 70 percent of the non-last-born. 

Mother’s age at birth is about two years older for last-born children compared with non-last-born 

children. Surprisingly, preceding birth-to-birth intervals are much longer for last-born children, with a 

mean 9.5 months greater (for children of birth order 2 or higher). Comparing last-born children with nonlast-born children for social characteristics, there are also surprising differences. Non-last-born children 

are more likely to live in rural areas, to have been born at home, to have mothers with less education, and 

to come from somewhat poorer households. 

The study of the effects of succeeding inter-pregnancy intervals on children’s nutritional status requires 

that the children be born within the five years preceding the survey, and the next child (if any) would need 

to be conceived between the date of the index child’s birth and the date of the interview. For an index 

child age 54 months at the time of the survey, for example, the maximum subsequent interval would be 

approximately 43 months; for an index child age 18 months the maximum subsequent interval would be 

about 7 months. Thus there is a strong relationship between the age of the index child and the possible 

length of the subsequent interval. 

The older the index child, the longer the inter-pregnancy interval can be, with the mean length rising from 

10 to 32 months. On the other hand, chronic nutritional status and underweight increase with the age of 

children, up to about age 24 months. Therefore, younger children have shorter subsequent intervals on 

average and also are less likely to have chronic undernutrition, creating a strong bias in the analysis of the 

relationship between subsequent interval length and chronic nutritional status. 

The study of under-5 mortality in relation to the length of the succeeding intergenesic interval is fraught 

with potential bias from a shortened duration of breastfeeding and the desire to replace a child who died. 

Excluding all deaths of children who die before the next child is conceived can control for these biases. 

However, this control could introduce another source of bias: the longer the succeeding birth-toconception interval, the more deaths and deaths at older ages are excluded. The percent of under-5 deaths 

prior to a succeeding conception that are excluded from analyses increases as the interval lengthens. 
xii 

Slightly more than half of deaths are excluded when the interval is less than six months, increasing to 83 

percent for reference intervals of 36-47 months, and up to 96 percent for intervals of eight or more years. 

Beyond the usual considerations of omission of births and transference of births across questionnaire age 

boundaries, using data for last-born children or non-last-born children instead of data for all children born 

in the five years preceding the survey can result in biased research findings. Unfortunately, some DHS 

data are only collected for last-born children, and other data only for non-last-born children. The 

correction of one bias may create another bias, as in the estimation of mortality risks by succeeding 

interval. 

Further thought and research are needed on how to overcome these biases for analysis of last children and 

succeeding intervals. Given the likely negative effects on the health and well-being of children whose 

birth is followed quickly by another, substantial efforts should be made to try to overcome these biases. 
1 

1. Introduction and Purpose

Many studies have shown the deleterious relationships between the length of preceding inter-birth or 

inter-pregnancy intervals and the pregnancy, birth, and child that follows the interval. Outcomes that have 

been investigated include miscarriage and stillbirth; complications of pregnancy and delivery; low birth 

weight; neonatal; infant; and child mortality; and poor nutritional status. 

In a review of 58 observational studies, Conde–Agudelo and colleagues identified several plausible causal 

mechanisms, including folic acid deficiency, cervical insufficiency, vertical transmission of infections, 

inadequate breastfeeding, and sibling competition (Conde–Agudelo, Rosas–Bermúdez, and Kafury–Goeta 

2006). For an in–depth discussion of the literature on this relationship, see previous studies by Rutstein 

(Rutstein 2005; Rutstein 2008). Several studies demonstrated that even after adjusting for a variety of 

confounding factors, the effect of birth interval on young child mortality persists (Alam 1995; Alam and 

David 1998; Bhalotra and van Soest 2006; Conde–Agudelo, Rosas–Bermúdez,and Kafury–Goeta 2006; 

DaVanzo et al. 2008; Hosseinpoor et al. 2006; Koenig et al. 1990; Miller et al.1992; Mozumder et al. 

1998; Zenger 1993). 

In addition to the effects of preceding birth interval on child survival, studies have documented a strong 

relationship between the length of the preceding birth interval and chronic and general undernutrition 

such that nutrition outcomes are poorer for children with shorter preceding birth intervals (Dewey and 

Cohen 2007; Rutstein 2005). A recent study by Rutstein and Winter (2014) demonstrated the deleterious 

effects of combinations of preceding intergenesic intervals, birth order, and mother’s age at birth on 

childhood mortality and nutritional status. 

The conception and birth of another child after a short period of time would also seem to negatively affect 

the earlier-born child at the start of the intergenesic interval. The earlier child’s breastfeeding could be cut 

short, the mother may not have a chance to recover from the earlier pregnancy, birth, and breastfeeding, 

and care and attention given to the earlier child may be reduced. These considerations have led 

researchers to attempt to use data from the Demographic and Health Surveys (DHS) to investigate health 

outcomes linked to the subsequent birth or pregnancy interval. However, the nature of the problem and 

the structure of DHS data can lead to serious biases in these studies. It is the purpose of this paper to 

describe and illustrate some of these biases, which are hard to overcome in these kinds of data sets. 

Appendix Table A1 describes the types of data that are available for children and their restriction by age 

(or time since birth), birth order, survival status, and other data type. 

First, the use of information pertaining to the most-recently-born (last-born) child in the five years 

preceding the survey is compared with that of all children born in the most recent five-year period, and 

with children born in the fifteen-year period preceding the survey. Also, children born in the five years 

preceding the survey with a younger sibling or with the conception of a younger sibling (i.e., those with a 

succeeding interval) are compared with all children born in the most recent five-year period. Second, 

factors affecting the nutritional status of children under age 5 that are related to succeeding intervals are 

studied to evaluate their possibilities of causing bias in studies relating succeeding intervals to the 

nutritional status of children under age 5. Third, biases that may be present in the relationship between 

mortality of children under age 5 and succeeding intervals are examined. 

3 

2. Characteristics of Most-Recently-Born Children and 

Non-Most-Recently-Born Children Compared with All 

Children Born in the Most Recent Five-Year Period

The demographic and social characteristics of most-recently-born children differ from those of all 

children born in the five-year year period preceding the survey. Studied here are birth order, sex, age at 

time of survey, mother’s age at birth, length of the preceding birth interval, survival of the preceding 

child, area of residence, delivery in a health facility, mother’s level of education, and household wealth. 

Table 1 presents selected demographic and social characteristics of children born in the five years 

preceding the survey and alive at the time of the survey by whether the child is the most recently born. Of 

the near half a million children born in the preceding five years in 45 recent DHS surveys,1

 over 70 

percent are the most recently born (the last birth). These children differ somewhat in many ways from 

under-5 children who are not the last birth. However, due to the large representation of last-born children 

among all under-5 children, differences between the last-born and all children are lower. Therefore, the 

discussion will focus on the differences between last-born and non-last-born children. 

Last-born children are slightly less likely than non-last-born children to be of low birth order. Forty-seven 

percent of last-born children are of birth orders 1 or 2 compared with 51 percent of non-last-born children. 

Mean birth order is 0.2 children higher among the last-born. Last-born children are more likely to be male 

than the non-last-born, 51.6 versus 50.0 percent, respectively. This result is surprising in that the sex ratio 

of non-last-born children is 100. Somewhat less surprising is that there are more males among the lastborn (sex ratio of 107). This difference may be due to a preference for male children among couples who 

stop having children. For all children under age 5, the sex ratio is 105, the expected sex ratio at birth. 

Last-born living children tend to be younger than non-last-born children. While there is an approximate 

balance of all children under age 5, at around 20 percent for each single year of age, only about 24 percent 

of last-born children are age 3-4 compared with 70 percent of non-last-born children. This striking 

difference may cause substantial bias when analyses select just last-born or non-last-born children instead 

of all children. The time since birth also shows that non-last-born children were born about 1.7 years 

earlier than last-born children. The difference between the current age and time since birth distributions is 

due to the greater percentage of deaths among non-last-born children, at 11 percent versus 4 percent for 

last-born children. 

The age at birth of the mother is somewhat older for last-born children, by about two years, compared 

with that of the mothers of non-last-born children. Forty-three percent of last-born children were born to 

mothers under age 25 compared with 55 percent of non-last-born children. 

Preceding birth-to-birth intervals are surprisingly much longer for last-born children, with a mean 9.5 

months greater (for children of birth order 2 or higher). This difference may also bias results of analyses if 

 

1 Albania 2008-09, Armenia 2010, Azerbaijan 2006, Bangladesh 2011, Benin 2006, Bolivia 2008, Burkina Faso 

2010, Burundi 2010, Cambodia 2010, Cameroon 2011, Colombia 2010, Congo Democratic Republic 2007, 

Dominican Republic 2007, Egypt 2008, Ethiopia 2011, Ghana 2008, Guyana 2009, India 2005-06, Indonesia 2007, 

Jordan 2007, Kenya 2008-09, Lesotho 2009, Liberia 2007, Madagascar 2008-09, Malawi 2010, Mali 2006, 

Mozambique 2011, Namibia 2006-07, Nepal 2011, Niger 2006, Nigeria 2008, Pakistan 2006-7, Peru 2012, 

Philippines 2008, Rwanda 2010, Sao Tome and Principe 2008-09, Senegal 2010-11, Sierra Leone 2008 ,Swaziland 

2006-07, Tanzania 2010, Timor-Leste 2009 ,Uganda 2011, Ukraine 2007, Zambia 2007, and Zimbabwe 2010-11
4 

only one of the two groups of children is taken to represent all children under age 5.2 However, the 

percentage of children whose next older sibling died is not much different between the two groups. 

Comparing last-born children with non-last-born children for social characteristics, there are surprising 

differences. Non-last-born children are more likely to live in rural areas, are more likely to have been 

born at home instead of in a health facility, have mothers with lower levels of education, and come from 

somewhat poorer households. The differences between last-born children and non-last-born children may 

be partly due to differences in fertility levels between countries and areas within countries, as in higher 

fertility areas (such as rural areas) women are more likely to have had more than one child in the five 

years preceding the survey. 

It is therefore seen that selecting just last-born children or just non-last-born children among the children 

under age 5 may result in several sources of bias if it is assumed that the results would be the same as for 

all children. Due to the preponderance of last-born children among all children, the biases in using lastborn children are less than those in using non-last-born children. However, the type of analysis 

undertaken and data available may require using one group or the other instead of all children under age 

5. For example, DHS now collects many prenatal and postnatal care indicators only for last-born children. 

Given that far fewer births of non-last-born children are delivered in a health facility, biased conclusions 

may result. Dietary information is collected for almost no non-last-born children. If these children were 

weaned early, their nutritional status could be different from those of last-born children.

 

2

 The lengths of the succeeding birth-to-birth intervals cannot be compared between the groups since last-born 

children have no such interval. 
5 

Table 1. Children born in the five years preceding the survey living at the time of the survey, 45 

DHS surveys

All children 

Last-born 

children 

Non-last-born 

children

Number 433,073 308,724 124,349

Percent 100.0 71.3 28.7

 

Birth order 

1 26.0 24.4 30.1

2 22.0 22.4 20.8

3 16.0 16.3 15.1

4 11.5 11.7 11.1

5 8.2 8.2 8.2

6 5.9 6.0 5.7

7+ 10.4 11.0 9.0

Mean 3.3 3.3 3.1

Sex of child 

Male 51.1 51.6 50.0

Female 48.9 48.4 50.0

Current age of child (years) 

0 19.8 27.7 0.0

1 18.7 25.2 2.6

2 18.6 19.5 16.1

3 18.8 13.6 31.8

4 18.1 10.0 38.1

Died 6.1 3.9 11.4

Time since birth (years) 

0 20.5 28.8 0.0

1 19.8 26.3 3.6

2 19.9 20.3 18.8

3 20.2 14.2 35.3

4 19.5 10.4 42.2

Mean 2.0 1.5 3.2

Mother’s age at birth 

<18 6.9 6.0 9.3

18-24 39.7 37.3 45.6

25-29 40.8 42.2 37.4

30-39 9.0 10.1 6.1

40+ 3.6 4.5 1.5

Mean 26.1 26.6 24.7

Preceding birth-to-birth interval 

<24 months 15.3 13.6 19.5

24-35 months 24.6 23.7 26.8

36-47 months 14.7 15.4 13.0

48+ months 19.4 23.0 10.6

First birth-no preceding interval 26.0 24.4 30.1

Mean 40.9 43.5 34.0

(Continued...)
6 

Table 1. – Continued

All children 

Last-born 

children 

Non-last-born 

children

Preceding child death 

Percent with a death 5.8 5.5 6.7

Percent without a death or no preceding child 94.2 94.5 93.3

Succeeding birth-to-birth interval 

<24 months 10.0 na 10.0

24-35 months 12.7 na 12.7

36-47 months 5.0 na 5.0

48+ months 1.0 na 1.0

Most recent birth-no succeeding interval 71.3 na 71.3

Mean 27.8 na 27.8

Area of residence 

Urban 33.2 35.3 28.0

Rural 66.8 64.7 72.0

Place of child’s birth 

Non-medical 42.5 39.4 50.4

Medical 57.5 60.6 49.6

Mother’s education in years 

0 35.3 32.6 41.9

1-6 27.9 27.6 29.9

7-12 30.6 32.8 24.9

12+ 6.2 7.0 4.3

Mean 5.0 5.3 4.1

Wealth Index 

Value 

Mean -0.2 -0.1 -0.3

Median -0.4 -0.3 -0.5

Quintiles 

Poorest 24.5 23.6 28.4

Poorer 21.4 20.9 22.7

Middle 20.2 19.8 19.9

Richer 18.5 18.6 16.8

Richest 15.4 17.1 12.2
7 

3. Biases in the Effects of Subsequent Inter-Pregnancy 

Intervals on Nutritional Status and Mortality under Age 5

3.1 Nutritional Status of Children under Age 5 

The study of the effects of succeeding inter-pregnancy intervals looks at children’s nutritional status 

according to the length of time between the index child’s birth and the conception of the next live-born 

child, if any. Given that children studied need to have been born within the five years preceding the 

survey, the next child (if any) would need to be conceived between the date of the index child’s birth and 

the date of interview. For an index child age 54 months, the maximum subsequent interval would be 

approximately 43 months; for an index child age 18 months, the maximum subsequent interval would be 

about 7 months. Thus there is a very strong relationship between the age of the index child and the length 

of the subsequent interval. Table 2 demonstrates this relationship.

Table 2. Birth interval by age of child

Current age of child in years N 

Succeeding birth interval 

Minimum Maximum Mean

0 8 9 11 10.00

1 1,932 9 23 16.43

2 12,532 9 35 23.30

3 24,460 8 47 28.34

4 29,155 8 59 31.52

The older the index child, the longer can be the inter-pregnancy interval, with the mean length rising from 

10 to 32 months. On the other hand, chronic nutritional status and underweight increase with the age of 

children up to about age 24 months. In Figure 1, children’s height-for-age z-score decreases with 

increasing age until about age 20 months. In Figure 2 the percentage of children who are stunted increases 

with the child’s age up to about 24 months. Therefore, younger children have shorter subsequent intervals 

on average and also have less chronic undernutrition, creating a strong bias in the analysis of the 

relationship between subsequent interval length and chronic nutritional status. 
8 

<3 months 3-5 6-8 9-11 12-17 18-23 24-35 36-47 48-59

-2

-1.8

-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

Figure 1. Height for age z-score (WHO) by age of child

Age of child in months

Mean height for age zscore 

<3 months 3-5 6-8 9-11 12-17 18-23 24-35 36-47 48-59

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

Figure 2. Proportion stunted by age of child

Age of child in months

Proportion Stunted

Figure 1. Mean height-for-age z-score (WHO), by age of child

Figure 2. Proportion stunted, by age of child

Mean height for age z score 

Age of child in months
9 

Table 3 below summarizes the confounding that occurs between succeeding interval and chronic 

undernutrition by age of the index child. Children younger than 18 months have no or almost no 

succeeding births. For children 18 months or older, the proportion with a succeeding birth increases 

rapidly, as do the proportions for children with a succeeding birth interval between 12 and 23 months and 

between 24 and 35 months. However, there are no birth intervals 48 months or longer. The mean interval 

length also increases rapidly with the child’s age. As noted above, the average height-for-age z-score 

decreases and the percentage stunted increases with the child’s age, creating a serious confounder for 

examining the relationship between succeeding interval and chronic nutritional status.

Table 3. Percentage of children last born and with succeeding birth-to-birth intervals, mean 

succeeding interval length, height-for-age standard deviation, and percentage stunted by child’s 

age, for living children born in the five years preceding the survey, 40 DHS surveys (255,760 

children)

 Child’s age in groups

 <3 

months 

3-5 

months 

6-8 

months

9-11 

months

12-17 

months

18-23 

months

24-35 

months

36-47 

months 

48-59 

months Total

Child is last-born 100% 100% 100% 100% 99% 94% 75% 52% 40% 73%

Following birth to 

birth interval less 

than 12 months 

0% 0% 0% 0% 1% 5% 7% 8% 9% 5%

Following birth to 

birth interval 12 

to 23 months 

0% 0% 0% 0% 0% 1% 17% 25% 25% 13%

Following birth to 

birth interval 24 

to 35 months 

0% 0% 0% 0% 0% 0% 1% 15% 18% 25%

Following birth to 

birth interval 48 

or more months 

0% 0% 0% 0% 0% 0% 0% 0% 0% 0%

Succeeding birth 

interval 

 10.0 13.3 17.2 23.3 28.3 31.5 28.4

 

Ht/A standard 

deviations 

(according to 

WHO) 

-0.31 -0.42 -0.63 -0.93 -1.39 -1.70 -1.72 -1.68 -1.59 -1.42

Stunted (WHO) 15% 16% 19% 24% 35% 43% 42% 40% 36% 35%

Another serious confounder is breastfeeding. Unless they are postpartum abstinent or using postpartum 

contraception, mothers who stop breastfeeding are more likely to have a short succeeding interval. The 

nutritional impact will depend on what age the weaning occurred and what foods the child received after 

weaning. On the other hand, women who become pregnant while breastfeeding are very likely to stop, 

and so short succeeding intervals may cause early weaning. Most DHS surveys do not possess data on the 

duration of breastfeeding of the index child with a succeeding interval, and the early DHS surveys that 

asked about duration of breastfeeding of non-last-born children had data that were severely heaped, 

making them useless for analysis. 
10 

3.2 Under-5 Mortality by Succeeding Birth-to-Conception Interval

The study of under-5 mortality in relation to the length of the succeeding intergenesic interval is fraught 

with potential for bias. One such bias relates to breastfeeding. Children who stop breastfeeding early in 

life face additional mortality risks due to malnutrition and increased prevalence of disease, including 

diarrhea and acute respiratory infection, especially in low-income settings where milk substitutes are 

inadequately given and quality health services may not be frequently used. Shortened durations of 

breastfeeding lead to the early return of postpartum fecundity and are a cause of a short time until the next 

pregnancy where adequate postpartum contraception and/or abstinence are not practiced. Children who 

die at young ages also lead to shorter periods of postpartum lactational amenorrhea and thus short 

succeeding intervals. Another potentially confounding factor is the desire to replace the child who died. 

Families may want to have another child as soon as possible when a young child dies and thus may not 

use contraception or prolong periods of postpartum abstinence. 

One way to control for these breastfeeding and replacement-desire effects is to exclude all deaths of 

children who die before the next child is conceived. A child who dies before the next is conceived cannot 

have been affected by that conception. However, this control could be introducing an additional source of 

bias: the longer the succeeding birth-to-conception interval, the more deaths and deaths at older ages are 

excluded. Given that the risk of mortality declines as age at exposure increases, there is a double lowering 

of mortality of longer intervals compared with shorter intervals. This phenomenon may be the cause of 

the following results. 

Figure 3 shows the adjusted risk of child death by birth-to-conception interval relative to that of the 

reference interval (36-47 months), using pooled data from 52 DHS surveys for the 15-year period 

preceding the survey. The relative risk ratio has been adjusted by including a host of characteristics in a 

Cox hazard regression.3

 Due to the large selection bias that underestimates children’s deaths in the 

reference interval and longer intervals, the relative risks of dying for children with shorter intervals are 

too high when compared with those of the reference period, giving a misleading impression of the 

mortality effects of short succeeding intervals. 

 

3 Controls include sex of child, multiplicity of birth, birth order, mother’s age at birth, mother’s level of education, 

type of area of residence, level of household wealth, source of water, type of toilet, and possession of a refrigerator. 
11 

Table 4 shows the percentage of under-five deaths prior to conception of the succeeding child that are 

excluded from analyses, by succeeding interval for children born in the 15-year period preceding the 

survey. When the interval is less than six months, slightly more than half of under-5 deaths that occurred 

prior to conception of the succeeding child are excluded (column 4). These deaths would be mainly 

neonatal deaths. As the interval lengthens, the percent of under-5 deaths that are excluded increases, 

reaching 83 percent for the 36-47 month reference interval, and up to 96 percent for intervals of eight 

years (96 months) or more. 

Another way of viewing this bias is the ratio of deaths by interval length to that of the reference category. 

For an interval of less than 6 months between birth and the succeeding conception, the proportion of 

deaths included is almost three times higher compared with the reference category, and twice as high for 

intervals of 12 to 17 months (column 5).

Figure 3. Child mortality by length of succeeding birth interval, children born 1 to 15 

years prior to survey, 52 DHS surveys 2006-2012 
12 

Table 4. Excluded deaths by length of succeeding interval, children born 1 to 15 years prior to 

survey, 52 DHS surveys 2006-2012

Harmonized 

succeeding birthto- succeedingconception 

interval (months) 

Death before succeeding conception 

Percent of 

deaths 

excluded 

Ratio of 

percent of 

deaths 

included

Death did not 

occur before 

succeeding 

conception 

Death occurred 

before 

succeeding 

conception Total 

<6 8,334 9,000 17,334 51.9 2.9

6 to 11 6,535 10,583 17,118 61.8 2.3

12 to 17 6,792 13,665 20,457 66.8 2.0

18 to 23 4,029 8,756 12,785 68.5 1.9

24 to 29 1,861 6,168 8,029 76.8 1.4

30 to 35 1,053 3,776 4,829 78.2 1.3

36 to 47 (ref.) 892 4,405 5,297 83.2 1.0

48 to 59 279 2,243 2,522 88.9 0.7

60 to 95 169 2,061 2,230 92.4 0.5

96+ 15 354 369 95.9 0.2

Last births 21,287 21,287 0.0 5.9

Total 51,246 61,011 112,257 54.3 2.7
13 

4. Conclusions and Further Research Needed

When researchers use the DHS surveys for the analysis of children born in the five years preceding the 

survey, they must be very careful to consider and avoid inherent biases that derive from both the data 

structure and the nature of the analysis. Beyond the usual considerations of omission of births and 

transference of births across questionnaire age boundaries, particularly the health section and calendar 

age/date boundaries, using data for last-born children or non-last-born children instead of all children 

born in the five years preceding the survey will probably result in biased research findings. Unfortunately, 

some DHS data are only collected for last-born children, for example, children’s dietary intake and 

several child health questions. Other types of research call for using non-last-born children, such as the 

analysis of current nutritional status by succeeding birth or pregnancy interval, because survey-measured 

nutritional status of children under age 5 is only relevant for those born in the last five years who have a 

succeeding interval—that is, non-last-born children under age 5. Also presented is the case that the 

necessary correction of one bias may incur the creation of another type of bias, as in the estimation of 

mortality risks by succeeding interval. 

Further thought and research are needed on how to overcome these biases for analysis of last-born 

children and succeeding intervals. Possible solutions are to obtain information on all children instead of 

just the last-born, collect past nutritional status information, as the Peru Continuous DHS survey does 

from health cards, and collect follow-up information using cohort panels. Other analytical techniques may 

also be helpful such as differential weighting, instrumental variable and two-stage regressions,4

 and timevarying covariation for death and next conception. Given the possibly great effects on the health and wellbeing of children whose birth is followed quickly by that of another, substantial efforts should be made in 

trying to overcome these biases. 

 

4 As an example, an analysis using two-stage regression could estimate the level of stunting according to the child’s 

age. Then the predicted level of stunting could be subtracted from the actual level for each child and the difference 

could be regressed on the succeeding birth-to-pregnancy interval. Control variables (education, wealth, etc.) could 

be introduced in either the first or the second regression. 

15 

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Appendix

Appendix Table A1. Types of DHS data available on children, by restriction

Questionnaire source of information Type of data 

Restrictions on data

Time since birth Survival Other

Household schedule Age, sex, relation to head All Living children 

Marital status 15+ years Living children Survival and residence of parents 0 to 17 years Living children 

Education attainment 5+ years Living children Education attendance 5 to 24 years Living children 

Birth registration 0 to 4 years Living children 

Household biomarker form Height, weight 0 to 5 years Living children 

Hemoglobin level 6 months to 5 years Living children 

Women’s questionnaire Birth history Date of birth, sex, multiplicity of birth, birth order, survival All All 

Preceding and succeeding birth interval, age of mother at birth All All Age at death All Dead children 

Maternity section Wantedness, size and weight at birth, delivery assistance 0 to 4 years All All

Place of delivery, caesarian delivery 0 to 4 years All AllAntenatal care 0 to 4 years All Last birth onlyTime in delivery facility, postnatal check of mother, baby 0 to 4 years All Last birth onlyPostpartum amenorrhea and abstinence 0 to 4 years All All

Ever breastfed, use of bottle with nipple 0 to 4 years All AllTime to first breastfeed and liquids in first 3 days 0 to 4 years All Last birth onlyStill breastfeeding 0 to 4 years Living children Last birth only

Child health section Vaccinations 0 to 4 years Living children All

Illness and treatment 0 to 4 years Living children All

Stool disposal 0 to 4 years Living children Youngest living with mother

Foods and liquids in last 24 hours 0 to 2 years Living children Youngest living with mother

Reproductive calendar Duration of pregnancy, prior termination 0 to 4 years All 

Birth as a result of contraceptive failure 0 to 4 years All 

17