NCHS Data Brief ■ No. 52 ■ December 2010

u.s. department of health and human services

Centers for Disease Control and Prevention

National Center for Health Statistics

Blood Lead and Mercury Levels in Pregnant Women in the 

United States, 2003–2008

Lindsey Jones, M.A.; Jennifer D. Parker, Ph.D.; and Pauline Mendola, Ph.D.

Chemical exposure during pregnancy is potentially harmful to the developing 

fetus, as the placenta cannot protect against heavy metals such as lead 

and mercury (1–4). Cord blood mercury levels have been associated with 

childhood cognitive function (3,4). High levels of lead exposure during 

pregnancy have been associated with adverse birth outcomes and, in some 

studies, with lower cognitive function test scores in childhood (3); relatively 

low lead levels have recently been associated with a small risk of decreased 

birthweight (5). While intervention for pregnant women with blood lead levels 

greater than or equal to 5 μg/dL has been recommended (6,7), no comparable 

recommendation for blood mercury in pregnant women has been established.

This report presents geometric mean lead and mercury blood levels of 

pregnant women in the United States based on using the 2003–2008 National 

Health and Nutrition Examination Surveys (NHANES). Throughout this 

report, the term mean refers to the geometric mean.

Keywords: pregnancy • National Health and Nutrition Examination Survey • 

environmental health

Pregnant women have lower mercury and lead levels than 

nonpregnant women.

Key findings

Data from the National 

Health and Nutrition 

Examination Survey, 

2003–2008

•	 In general, U.S. pregnant 

women have low levels of lead 

(less than 5 μg/dL) in their 

blood.

•	 Pregnant women have 

lower mercury and lead levels 

than nonpregnant women.

•	 Among pregnant women, 

mercury levels, but not lead 

levels, increase with age.

•	 Pregnant women with a 

prior pregnancy have lower 

mercury levels and higher lead 

levels than those without a 

prior pregnancy.

•	 Mexico-born MexicanAmerican pregnant women 

have higher lead levels than 

their U.S.-born counterparts, 

but similar mercury levels.

NOTE: Access data table for Figure 1 at: http://www.cdc.gov/nchs/data/databriefs/db52_tables.pdf#1.

SOURCE: 2003–2008 National Health and Nutrition Examination Survey. μg/L

μg/dL

0.0

0.2

0.4

0.6

0.8

1.0

Mercury 0.0

0.2

0.4

0.6

0.8

1.0

Lead

Pregnant Not pregnant

Figure 1. Age-adjusted mean blood mercury and lead levels in women aged 18–49 years, 

by pregnancy status: United States, 2003–2008
NCHS Data Brief ■ No. 52 ■ December 2010

■  2  ■

•	 Pregnant women experience physiological and metabolic changes over the course of 

pregnancy that may affect measured lead and mercury levels, so levels among nonpregnant 

women may not be directly comparable.

•	 Mean mercury levels among pregnant women are lower than those for nonpregnant women 

(0.69 μg/L compared with 0.82 μg/L), even after adjusting for the younger ages of pregnant 

women (Figure 1).

•	 Mean lead levels among pregnant women are lower than those for nonpregnant women 

(0.64 μg/dL compared wtih 0.85 μg/dL), even after adjusting for the younger ages of 

pregnant women.

Mercury levels among pregnant women increase with age.

•	 Mean mercury levels among pregnant women increase with age, from 0.56 μg/L for women 

aged 18–24 years to 1.01 μg/L for women aged 35 years and over (Figure 2). 

•	 Lead levels do not vary by age among pregnant women.

NOTE: Access data table for Figure 2 at: http://www.cdc.gov/nchs/data/databriefs/db52_tables.pdf#2.

SOURCE: 2003–2008 National Health and Nutrition Examination Survey.

Figure 2. Mean blood mercury and lead levels in pregnant women aged 18–49 years, by age group: 

United States, 2003–2008 μg/L

μg/dL

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Mercury

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Lead

18–24 25–29 30–34 35 and over

Mercury levels are higher and lead levels are lower for pregnant women 

with no prior pregnancy.

•	 Blood mercury levels are higher for pregnant women with no prior pregnancies (0.88 μg/L) 

than for women with one or more prior pregnancies (0.63 μg/L). This pattern is similar for 

pregnant women younger than age 30, but not statistically significant (Figure 3).

•	 Lead levels are lower for pregnant women with no prior pregnancy (0.56 μg/dL) than for 

women with one or more prior pregnancies (0.67 μg/dL). This pattern is similar for pregnant 

women younger than age 30.
NCHS Data Brief ■ No. 52 ■ December 2010

■  3  ■

NOTE: Access data table for Figure 3 at: http://www.cdc.gov/nchs/data/databriefs/db52_tables.pdf#3.

SOURCE: 2003–2006 National Health and Nutrition Examination Survey.

Figure 3. Mean blood mercury and lead levels in pregnant women aged 18–49 years, overall, and among women under 

age 30, by number of prior pregnancies: United States, 2003–2006 μg/L

μg/dL

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

No prior pregnancy One or more prior pregnancies 1.0

Mercury Lead

Overall Less than 30 years Overall Less than 30 years

Pregnant women with less than a high school education have higher lead 

levels than those with more education. 

•	 There are no statistically significant differences in mean mercury levels of pregnant women

25 years of age or over based on their educational level (Figure 4).

•	 Mean lead levels for pregnant women aged 25 years and over with less than a high school 

education (0.85 μg/dL) are higher than those for women with some college education (0.60 

μg/dL) and those for college graduates (0.57 μg/dL).

NOTE: Access data table for Figure 4 at: http://www.cdc.gov/nchs/data/databriefs/db52_tables.pdf#4.

SOURCE: 2003–2008 National Health and Nutrition Examination Survey.

Figure 4. Mean blood mercury and lead levels in pregnant women aged 25 and over, by educational attainment: 

United States, 2003–2008 μg/L

μg/dL

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

High school graduate or less Some college College graduate 1.0

Mercury Lead
NCHS Data Brief ■ No. 52 ■ December 2010

■  4  ■

Mercury and lead levels among pregnant women vary by race and ethnicity.

•	 Pregnant Mexican-American women have lower mean mercury levels than pregnant nonHispanic black women (0.56 μg/L compared with 0.76 μg/L) (Figure 5).

•	 Mean mercury levels for pregnant Mexican-American women are not different from those 

for pregnant non-Hispanic white women.

•	 Non-Hispanic white pregnant women have lower mean lead levels (0.53 μg/dL) than other 

race and ethnicity groups (non-Hispanic black: 0.73 μg/dL and Mexican American: 0.90 μg/dL).

NOTE: Access data table for Figure 5 at: http://www.cdc.gov/nchs/data/databriefs/db52_tables.pdf#5.

SOURCE: 2003–2008 National Health and Nutrition Examination Survey.

Figure 5. Mean blood mercury and lead levels in pregnant women aged 18–49 years, by race and ethnicity: 

United States, 2003–2008 μg/L

μg/dL

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Non-Hispanic white Non-Hispanic black Mexican American 1.0

Mercury Lead

Mexican-American pregnant women born in Mexico have higher lead levels 

than Mexican-American pregnant women born in the United States.

•	 Mexican-American pregnant women born in Mexico and the United States have similar 

mean mercury levels (Figure 6).

•	 Mean lead levels for Mexican-American pregnant women born in Mexico (1.08 μg/dL) are 

over 50% higher than for those born in the United States (0.62 μg/dL).
NCHS Data Brief ■ No. 52 ■ December 2010

■  5  ■

NOTE: Access data table for Figure 6 at: http://www.cdc.gov/nchs/data/databriefs/db52_tables.pdf#6.

SOURCE: 2003–2008 National Health and Nutrition Examination Survey.

Figure 6. Mean blood mercury and lead levels in Mexican-American pregnant women aged 18–49 years, by country of 

birth: United States, 2003–2008 μg/L

μg/dL

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Mercury

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Lead

United States Mexico

Summary

Based on a nationally representative sample, this analysis demonstrates that mean lead levels are 

generally low in U.S. pregnant women (less than 5 μg/dL for blood lead). Although no standard 

exists for blood mercury in pregnant women, based on studies of cord blood (4), levels greater 

than 5.8 μg/L have raised concern (8,9). The mean blood mercury level in pregnant women in

this analysis is 0.69 μg/L. Pregnant women experience physiological and metabolic changes 

over the course of pregnancy that may affect measured lead and mercury levels so levels among 

nonpregnant women may not be directly comparable. However, results for mercury are similar to 

previously published results by the Centers for Disease Control and Prevention (CDC) for women 

of childbearing age and for pregnant women using earlier NHANES data (8,10). Although there 

are no previous nationally representative studies published on lead levels in pregnant women, 

data from 10 states within the Adult Blood Lead Epidemiology and Surveillance (ABLES) system 

have reported the prevalence of blood lead levels greater than 5 μg/dL for women of childbearing 

age and higher blood lead levels among occupationally exposed women from 37 states (6); earlier 

studies from NHANES II reported levels of blood lead among women of childbearing age during 

1976–1980, when ambient lead levels were higher (11,12). In this analysis, so few pregnant 

women had blood lead levels that could be considered high (less than 0.5% in the weighted 

sample) that no reliable estimates for high levels could be calculated (relative standard errors are 

greater than 50%).

There were some observed differences among mothers by number of previous pregnancies, 

education level, race and ethnicity, and country of birth. Some of these differences could possibly 

be explained by differences in diet (e.g., fish consumption) (10,13) and other environmental exposures.
NCHS Data Brief ■ No. 52 ■ December 2010

■  6  ■

Definitions

Geometric mean: A type of average that is commonly used to estimate central tendency for 

data that are distributed with a long tail at the upper end of the distribution. In this report, the 

geometric mean was calculated by taking the natural logarithm of each concentration, computing 

the mean of those log-transformed values, and exponentiating the value to the original scale.

Pregnant: Defined as currently pregnant and determined by the use of either a lab pregnancy test

or self-reported pregnancy status.

Data source and methods

The 2003–2008 NHANES data were used for all analysis. NHANES selects a sample for the 

survey that is representative of the noninstitutionalized civilian U.S. population of all ages. 

A national sample of about 5,000 civilian, noninstitutionalized persons is surveyed annually. 

During 1999–2006, the NHANES sampling criterion included oversampling of pregnant women 

to produce reliable estimates of health and nutritional measures for this subgroup; data for the 

number of prior pregnancies are only available for 1999–2006. Therefore, estimates by number of 

prior pregnancies in this report are for 2003–2006 rather than 2003–2008. Consistent laboratory 

methods for mercury detection are available in NHANES from 2003 onward. The sample design 

utilizes a multistage stratified cluster design where first counties are randomly chosen, then within

county, segments (smaller geographical units) are randomly chosen followed by households 

within those units and lastly, one or more individuals is selected from each household. This report 

is based on an analysis of the household interview data and laboratory data files. Information

about laboratory methods can be found in CDC’s Fourth National Report on Human Exposure to 

Environmental Chemicals (14).

Because the distribution of age among pregnant women 18–49 years of age is lower than that for 

nonpregnant women in the same age range, lead and mercury levels for pregnant and nonpregnant 

women shown in Figure 1 were age-adjusted to the age distribution of births to women in 2004 

with the following weights: ages under 25 years, weight = 0.3543; ages 25–34 years, weight = 

0.5034; and ages greater than 34 years, weight = 0.1424.

Survey sample weights were used for all analysis, which accounts for differential probabilities 

of selection, nonresponse, and noncoverage. All data analysis was completed using SAS version 

9.1 (SAS Institute, Cary, N.C.) and SUDAAN version 9.0 (RTI, Research Triangle Park, N.C.). 

Confidence intervals were calculated and pair-wise statistical tests were conducted using the

log-transformed values. Statements about differences and similarities in the text are based on the 

results of statistical tests using p = 0.05 as the criterion for statistical significance; no adjustments

for multiple comparisons were made. None of the reported geometric mean estimates are 

considered unreliable; all have an estimated relative standard error less than 30%.
NCHS Data Brief ■ No. 52 ■ December 2010

■  7  ■

About the authors

Jennifer Parker is with CDC’s National Center for Health Statistics (NCHS), Office of Analysis

and Epidemiology (OAE). Pauline Mendola is with CDC, NCHS, OAE, Infant, Child, and 

Women’s Health Statistics Branch. At the time of this report, Lindsey Jones was a Graduate 

Research Assistant at the University of Maryland, School of Public Health, Department of 

Epidemiology and Biostatistics. Ms. Jones is currently a CDC/CSTE Applied Epidemiology 

Fellow at the Tennessee Department of Health.

References

1. Rhainds M, Levallois P, Dewailly E, Ayotte P. Lead, mercury, and organochlorine compound 

levels in cord blood in Quebec, Canada. Arch Environ Health 54(1):40–7. 1999.

2. Crocetti AF, Mushak P, Schwartz J. Determination of numbers of lead-exposed women of 

childbearing age and pregnant women: An integrated summary of a report to the U.S. Congress 

on childhood lead poisoning. Environ Health Perspect 89:121–4. 1990.

3. Wigle DT, Arbuckle TE, Turner MC, et al. Epidemiologic evidence of relationships between 

reproductive and child health outcomes and environmental chemical contaminants. J Toxicol 

Environ Health B Crit Rev 11:373–517. 2008.

4. Centers for Disease Control and Prevention. National Academy of Sciences. Toxicologic 

effects of methylmercury. Washington, D.C.: National Research Council; 2000. Lead exposure 

among females of childbearing age—United States, 2004. MMWR 56:387–400. 2007.

5. Zhu M, Fitzgerald EF, Gelberg KH, et al. Maternal low-level lead exposure and fetal growth. 

Environ Health Perspect 118:1471-5. 2010.

6. Centers for Disease Control and Prevention. Lead exposure among females of childbearing 

age: United States, 2004. MMWR 56:397-400. 2007.

7. Association of Occupational and Environmental Clinics. Medical management guidelines for 

lead-exposed adults. Washington, D.C.: Association of Occupational and Environmental Clinics. 

2007. Available from: http://www.aoec.org/documents/positions/mmg_final.pdf. Accessed

September 13, 2010.

8. Centers for Disease Control and Prevention. Blood mercury levels in young children and 

childbearing aged women: United States, 1999–2002. MMWR 53:1018–20. 2004.

9. U.S. Environmental Protection Agency. Methylmercury exposure. Available from: http://

www.epa.gov/mercury/exposure.htm. Accessed September 13, 2010.

10. Schober SE, Sinks TH, Jones RL, et al. Blood mercury levels in U.S. children and women of 

childbearing age, 1999–2000. JAMA 289:1667–74. 2003.

11. Geronimus AT, Hillemeier MM. Patterns of blood lead levels in U.S. black and white women 

of childbearing age. Ethnicity Dis 2:222–3. 1992.

12. Crocetti AF, Mushak P, Schwartz J. Determination of numbers of lead-exposed women of 

childbearing age and pregnant women: An integrated summary of a report to the U.S. Congress 

on childhood lead poisoning. Environ Health Perspect 89:121–4. 1990.
NCHS Data Brief ■ No. 52 ■ December 2010

ISSN 1941–4927 (Print ed.) 

ISSN 1941–4935 (Online ed.) 

CS218397 

T-38165 (12/2010) 

DHHS Publication No. (PHS) 2011–1209

13. McDowell MA, Dillon CF, Osterloh J, et al. Hair mercury levels in Suggested citation

U.S. children and women of childbearing age: Reference range data from Jones L, Parker JD, Mendola P. Blood lead NHANES 1999–2000. Environ Health Perspect 112:1165–71. 2004. and mercury levels in pregnant women in 

the United States, 2003–2008. NCHS data 14. Centers for Disease Control and Prevention. Fourth National Report brief no 52. Hyattsville, MD: National 

on Human Exposure to Environmental Chemicals. National Center for Center for Health Statistics. 2010.

Environmental Health, Division of Laboratory Sciences, Atlanta, GA. 2009. 

Available from: http://www.cdc.gov/exposurereport/. 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

Edward J. Sondik, Ph.D., Director

Jennifer H. Madans, Ph.D., Associate 

 Director for Science

Office of Analysis and Epidemiology 

Linda T. Bilheimer, Ph.D., Associate 

 Director

U.S. DEPARTMENT OF 

HEALTH & HUMAN SERvICES first class mail 

postage & fees paid 

Centers for Disease Control and Prevention cdc/nchs 

National Center for Health Statistics Permit No. G-284

3311 Toledo Road

Hyattsville, MD 20782

OFFICIAL BUSINESS 

PENALTy FOR PRIvATE USE, $300

To receive this publication regularly, contact the 

National Center for Health Statistics by 

calling 1–800–232–4636 

E-mail: cdcinfo@cdc.gov

Internet: http://www.cdc.gov/nchs