NCHS Data Brief ■ No. 240 ■ April 2016

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES

Centers for Disease Control and Prevention

National Center for Health Statistics

Short Interpregnancy Intervals in 2014: Differences by 

Maternal Demographic Characteristics 

Marie E. Thoma, Ph.D., Casey E. Copen, Ph.D., and Sharon E. Kirmeyer, Ph.D.

Key findings 

Data from the National 

Vital Statistics System

● Nearly 29.0% of U.S. 

mothers who had a second or 

higher-order birth in 2014 had a 

short interpregnancy interval of 

less than 18 months. 

● Short intervals (i.e., less 

than 6 months, 6–11 months, 

and 12–17 months) were more 

common for mothers aged 

35 and over (5.7%, 16.3%, 

and 22.1%, respectively) than 

mothers who were under age 20 

at their previous birth (5.1%, 

8.8%, and 8.4%). 

● Short intervals of less than 

6 months and 6–11 months 

were more common among 

non-Hispanic black mothers 

(7.1% and 11.7%, respectively) 

than non-Hispanic white 

mothers (4.1% and 11.2%) and 

Hispanic mothers (5.0% and 

9.3%). 

● The percentage of births 

to mothers with intervals less 

than 6 months decreased as 

education level increased, 

from 4.3% (no high school 

diploma) to 1.8% (doctorate or 

professional degree).

Short interpregnancy intervals are associated with adverse birth outcomes, 

such as preterm birth (1,2). The risk of adverse birth outcomes has been 

shown to increase as the duration of short intervals decrease (i.e., 12–17 

months, 6–11 months, and less than 6 months), and these patterns may reflect 

different maternal demographic profiles. This report examines categories of 

short interpregnancy intervals by maternal demographic characteristics among 

second and higher-order singleton births, using revised birth certificate data for 

47 states and the District of Columbia (96% of births) in 2014. 

Keywords: pregnancy intervals • birth spacing • birth certificate • NVSS

How common were short interpregnancy intervals among 

mothers in 2014?

● Overall, 28.9% of U.S. mothers who had a second or higher-order birth 

in 2014 had a short interval of less than 18 months (Figure 1).

 

http://www.cdc.gov/nchs/data/databriefs/db240_table.pdf#1 
NCHS Data Brief ■ No. 240 ■ April 2016

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● The median interpregnancy interval was 24–29 months, or 2–2.5 years (Figure 1).

● Interpregnancy intervals of 12–17 months (13.4%) were the most common of all short 

interval categories, followed by intervals of 6–11 months (10.7%) and 1–5 months (4.8%). 

● Less than 5.0% of births occurred to mothers with an interval of 10 years or more (i.e.,

120 months or more). 

Were older mothers more likely to have short interpregnancy intervals? 

● The percentages of births to mothers with intervals less than 6 months were higher for 

mothers aged 35 and over at the previous birth (5.7%) and for mothers under age 25 at 

the previous birth (5.1% among those under age 20 and 5.6% among age group 20–24) 

compared with other age groups (Figure 2). 

● The percentage of births to mothers with intervals of 6–11 months increased with increasing 

maternal age at previous birth: from 8.8% among mothers under age 20 to 16.3% among 

mothers aged 35 and over. 

● Similarly, the percentage of births to mothers with intervals of 12–17 months increased with 

increasing maternal age at previous birth: from 8.4% among mothers under age 20 to 22.1% 

among mothers aged 35 and over. 

 

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NCHS Data Brief ■ No. 240 ■ April 2016

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Were there differences in the percentage of short interpregnancy intervals 

by race and Hispanic origin?

● Intervals of less than 6 months were most common among non-Hispanic black mothers 

(7.1%), followed by Hispanic (5.0%) and non-Hispanic white (4.1%) mothers (Figure 3).

● Intervals of 6–11 months were most common among non-Hispanic black mothers (11.7%), 

followed by non-Hispanic white (11.2%) and Hispanic (9.3%) mothers. 

● In contrast, intervals of 12–17 months were most common among non-Hispanic white 

mothers (15.6%), followed by non-Hispanic black (11.0%) and Hispanic (10.3%) mothers.

 

 

http://www.cdc.gov/nchs/data/databriefs/db240_table.pdf#3 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 
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Did the percentage of short interpregnancy intervals differ by maternal 

education level? 

● The percentage of births to mothers with intervals less than 6 months decreased with 

increasing education, from 4.3% among mothers with no high school diploma to 1.8% 

among mothers with a doctorate or professional degree (Figure 4). 

● The percentages of births to mothers with intervals of 6–11 months by education level were 

higher for those with a bachelor’s degree (10.3%), master’s degree (9.9%), or doctorate or 

professional degree (10.3%) compared with other education groups (8.7% for some college 

or associate’s degree, 8.4% for high school diploma, and 8.3% for no high school diploma).

● Differences by education level were most pronounced for intervals of 12–17 months: from 

9.3% of mothers with no high school diploma to 20.1% of mothers with a doctorate or 

professional degree. 

 

1

Differences are significant with each increasing education level except that master’s degree is not significantly different from doctorate or professional degree. 2

Differences are significant between bachelor’s, master’s, and doctorate or professional degrees compared with some college/associate’s degree, high school 

diploma, and no high school diploma.

NOTES: Interpregnancy interval is the number of months between a live birth and the conception of the next live birth. Analysis is restricted to maternal age at 

recent birth of 25 and over to account for the minimum years of schooling needed to obtain a doctorate or other professional degree. Access data table for

Figure 4 at: http://www.cdc.gov/nchs/data/databriefs/db240_table.pdf#4.

SOURCE: CDC/NCHS, National Vital Statistics System. 
NCHS Data Brief ■ No. 240 ■ April 2016

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Summary

Among mothers with a previous live birth, almost one-third of births in 2014 were conceived 

within 18 months, and nearly 5% were conceived within 6 months of the previous live birth. 

Interpregnancy intervals less than 6 months were more common among mothers who were 

younger (under age 25) or older (aged 35 and over) at the time of their previous birth, mothers 

who were non-Hispanic black, and mothers with lower education. By comparison, intervals of 

6–17 months were more likely to occur among mothers who were older (aged 35 and over) at 

the time of their previous birth and who had higher education. Non-Hispanic white mothers were 

more likely to have intervals of 12–17 months, whereas non-Hispanic black mothers were more 

likely to have intervals of 6–11 months, compared with other race and Hispanic-origin groups. 

Patterns of maternal demographic characteristics varied across short interval categories, 

suggesting that, even among intervals less than 18 months, differences in demographic patterns 

exist depending on interval length. Health care programs can use this report to better understand 

these differences and help target strategies to reduce the occurrence of short interpregnancy 

intervals (3).

Definitions

Interpregnancy interval: The number of months between a live birth and the conception of the 

next live birth. This was calculated by subtracting the “Date of last live birth” item from the 

date of birth to obtain a live-birth interval, then subtracting gestational age (months) of the birth 

from the live-birth interval. More detailed information on the calculation of this measure, which 

was shown to be comparable with national data from the National Survey of Family Growth, is 

provided in a previous report (4). 

Short interpregnancy interval: The conception of a live birth occurring less than 18 months from 

the preceding live birth (2). Short interpregnancy intervals were categorized as less than 6 months 

(i.e., 1–5 months), 6–11 months, and 12–17 months. 

Race and Hispanic origin: Hispanic origin and race are reported separately on the birth certificate. 

Race categories are consistent with the 1997 Office of Management and Budget standards (5). 

Data for Hispanic origin include all persons of Hispanic origin of any race. Findings are presented 

for the largest single-race and Hispanic-origin groups.

Maternal age at previous birth: The age of the mother at the time of her previous birth was 

calculated by subtracting the mother’s calculated interpregnancy interval from the mother’s 

current age at birth in 2014. For example, if the mother was aged 30 at the birth of her second 

child in 2014 and had an interpregnancy interval of 2 years, then her maternal age at the previous, 

or first, birth would be 28. 
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Data source and methods

This report contains 2014 data from the Natality Data Files of the National Vital Statistics 

System (NVSS). The NVSS natality files (available from: http://www.cdc.gov/nchs/data_access/

Vitalstatsonline.htm) include information on a wide range of maternal and infant demographic 

and health characteristics for all births occurring in the United States. Final data may also be 

accessed from the interactive data access tool VitalStats, available from: http://www.cdc.gov/

nchs/VitalStats.htm.

Data in this report are based on 100% of births to residents of 47 states and the District of 

Columbia that implemented the 2003 revision of the U.S. Standard Certificate of Live Birth as of 

January 1, 2014 (96% of 2014 U.S. births). Although the 47-state area may not be representative 

of the overall U.S. population because it is not a random sample of U.S. births (6), a previous 

study showed comparable reporting of interpregnancy intervals using birth certificate data in a 

limited reporting area with national data on interpregnancy intervals using the National Survey of 

Family Growth (4). 

The data presented are based on second or higher-order singleton births. Bivariate associations 

presented may be explained by other factors not controlled for in the figures or not included in the 

report. Two-tailed z tests at the 0.05 level were used to evaluate differences between percentages 

for comparisons. 

About the authors

Marie E. Thoma, Casey E. Copen, and Sharon E. Kirmeyer are with CDC’s National Center for 

Health Statistics, Division of Vital Statistics, Reproductive Statistics Branch.

References

1. Conde-Agudelo A, Rosas-Bermúdez A, Kafury-Goeta AC. Birth spacing and risk of adverse 

perinatal outcomes: A meta-analysis. JAMA 295(15):1809–23. 2006.

2. Office of Disease Prevention and Health Promotion, U.S. Department of Health and Human 

Services. Healthy People 2020: Family planning objectives, FP–5: Reduce the proportion 

of pregnancies conceived within 18 months of a previous birth. Available from: http://www.

healthypeople.gov/2020/topics-objectives/topic/family-planning/objectives.

3. Thiel de Bocanegra H, Chang R, Howell M, Darney P. Interpregnancy intervals: Impact of 

postpartum contraceptive effectiveness and coverage. Am J Obstet Gynecol 210(4):311.e1–8. 

2014.

4. Copen CE, Thoma ME, Kirmeyer S. Interpregnancy intervals in the United States: Data from 

the birth certificate and the National Survey of Family Growth. National vital statistics reports; 

vol 64 no 3. Hyattsville, MD: National Center for Health Statistics. 2015.

5. U.S. Office of Management and Budget. Revisions to the standards for the classification of 

federal data on race and ethnicity. Fed Regist 62(210):58782–90. 1997.

6. National Center for Health Statistics. User guide to the 2014 Natality Public Use File. 

Hyattsville, MD. Available from: http://www.cdc.gov/nchs/data_access/vitalstatsonline.htm.
NCHS Data Brief ■ No. 240 ■ April 2016

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NCHS Data Brief ■ No. 240 ■ April 2016

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Suggested citation

Thoma ME, Copen CE, Kirmeyer SE. 

Short interpregnancy intervals in 2014: 

Differences by maternal demographic 

characteristics. NCHS data brief, no 240. 

Hyattsville, MD: National Center for Health 

Statistics. 2016.

Copyright information

All material appearing in this report is in 

the public domain and may be reproduced 

or copied without permission; citation as to 

source, however, is appreciated.

National Center for Health 

Statistics

Charles J. Rothwell, M.S., M.B.A., Director

Nathaniel Schenker, Ph.D., Deputy Director

Jennifer H. Madans, Ph.D., Associate 

Director for Science

Division of Vital Statistics

Delton Atkinson, M.P.H., M.P.H., P.M.P., 

Director

Hanyu Ni, Ph.D., M.P.H., Associate Director 

for Science

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