Despite overall declines in environmental sources, lead exposure remains an important public health problem for certain groups, including women of childbearing age, due to the potential exposure to the developing fetus and breastfeeding infant (CDC 2010). Women of childbearing age (15–49 y old) represent 23% of the total U.S. population and 46% of females (U.S. Census Bureau 2017), and, at any given time, about 9% are pregnant (Crocetti et al. 1990). Lead readily crosses the placenta by passive diffusion, and fetal blood lead concentrations are highly correlated with maternal blood lead concentrations (Goyer 1990). Prenatal lead exposure has known influences on maternal health, infant birth, and neurodevelopmental outcomes across a wide range of exposure levels (Bellinger 2005). No safe level of exposure to lead in children has been identified, and toxic effects of lead have been identified even at lower levels of exposure than previously considered harmful (NTP 2012). In addition, declines in environmental sources of lead highlight maternal bone as a continuing endogenous source of exposure (Hu and Hernandez-Avila 2002). Bone lead stores are mobilized during periods of increased bone turnover, such as during pregnancy and lactation (Gulson et al. 2016b, 2003), and lead is released into maternal blood and breastmilk (Ettinger et al. 2014; Gulson et al. 2016a). Since cumulative bone lead stores persist for years to decades, women and their infants may be at risk for lead exposure long after environmental sources have been abated.

In general, U.S. population lead exposures, which are estimated by blood lead levels (BLLs) measured in the National Health and Nutrition Examination Survey (NHANES), have declined over time (Raymond et al. 2014). This is due, in large part, to successful policies aimed at controlling sources of lead in the environment, including removal of lead from gasoline, paint, plumbing fixtures, and consumer products (Brown and Margolis 2012). However, numerous lead hazards still exist, including deteriorating lead-based paint and dust in housing built before the 1978 ban on lead in residential paint, as well as from drinking water provided by pipes made from lead, lead solder, and plumbing fixtures containing lead. Occupational and take-home exposures from workers or household members exposed to lead in the workplace remain an important preventable source of lead exposure, as occupational exposure accounts for over 90% of known lead exposure among adults (Alarcon 2016). Other preventable sources of lead exposure include soil, traditional/folk medicines, fishing sinkers, bullets, materials used to make ceramics and stained glass (Alarcon 2016), and consumer products, including certain foods, spices, medicines, cosmetics, and vitamins (Pfadenhauer et al. 2016).

Little is known about the full extent of lead exposure among U.S. women of childbearing age and, more importantly, among pregnant and lactating women who may pass lead to their unborn baby or breastfeeding infant, since adult women are not routinely tested (CDC 2010). Some population subgroups may be highly exposed, particularly foreign-born recent immigrants (Graber et al. 2006; Klitzman et al. 2002; Pezzi et al. 2019), workers in several high-risk occupations (Alarcon 2016; CDC 2007), and those practicing high-risk behaviors, such as pica of lead-contaminated objects, smoking or exposure to secondhand smoke, or renovation of older homes (CDC 2010; Richter et al. 2013; Thihalolipavan et al. 2013). Women living near hazardous waste sites or active smelters (García Vargas et al. 2001) and who are residents in countries where leaded gasoline is used or only has recently been phased out may also be exposed to relatively high levels of lead in the environment (Bede-Ojimadu et al. 2018; Forsyth et al. 2018; Prihartono et al. 2019).

We analyzed 40 y of NHANES data (1976–2016) to describe the distribution of BLLs in U.S. women of childbearing age (15–49 years of age) and the association with sociodemographic, reproductive, smoking, and housing characteristics.

Table 1 shows the sample size by NHANES period for all women 15–49 years of age, all women 15–49 years of age with available BLLs, women with BLLs ≥5μg/dL, and all women 15–49 years of age with available BLLs and current pregnancy status. Approximately 4.5–16.5% of women 15–49 years of age with complete blood lead data were currently pregnant at the time of survey for each of the survey periods. Figure 1 shows the shift in distribution of BLLs among U.S. women of childbearing age over the 40-y period from 1976 to 2016.

BLLs were highest among women 15–49 years of age in NHANES II (1976–1980). During this period, the GM BLL was 10.37μg/dL (95% CI: 9.95, 10.79), and 98.3% of women of childbearing age had BLLs ≥5μg/dL (95% CI: 97.1, 99.2), 61.4% had BLLs ≥10μg/dL (95% CI: 55.8, 66.8), and 3.6% had BLLs ≥20μg/dL (95% CI: 2.1, 5.5) (Table 2).

GMs, sample size, and estimated prevalence of BLL ≥5μg/dL by select characteristics are presented in Tables 3 and 4. BLLs were higher among women 15–49 years of age in NHANES III (1988–1991, 1991–1994) than in more recent continuous survey years (1999–2016). From 1991 to 1994, the GM BLL was 1.53μg/dL (95% CI: 1.45, 1.60), 3.8% had BLLs ≥5μg/dL (95% CI: 2.4, 5.6), and 0.3% had BLLs ≥10μg/dL (95% CI: 0.2, 0.5) (Table 3). From 1988 to 1991, the GM BLL was 1.83μg/dL (95% CI: 1.75, 1.94), 6.6% had BLLs ≥5μg/dL (95% CI: 5.4, 7.9), and 0.4% had BLLs ≥10μg/dL (95% CI: 0.1, 0.9) (Table 3).

From 1999 to 2002 (Table 4), the GM BLL was 1.05μg/dL (95% CI: 1.01, 1.09) in women 15–49 years of age, and 1.3% of women of childbearing age (15–49 y) had BLLs ≥5μg/dL (95% CI: 0.7, 2.0). Higher BLLs were associated with older age, non-Hispanic black, or Mexican-American race/ethnicity, birthplace outside of the United States, ever being pregnant, a higher number of live births, never breastfed, lower PIR (<1.3), not being currently pregnant, current smoking cigarettes, and secondhand smoke exposure.

From 2003 to 2006 (Table 4), the GM BLL was 0.91μg/dL (95% CI: 0.88, 0.95) in women 15–49 years of age, and 0.7% of women of childbearing age (15–49 y old) had BLLs ≥5μg/dL (95% CI: 0.4, 1.1). Higher BLLs were associated with older age, birthplace in Mexico, ever pregnant, a higher number of live births, never breastfed, and not currently pregnant.

From 2007 to 2010 (Table 4), the GM BLL was 0.81μg/dL (95% CI: 0.79, 0.84) in women 15–49 years of age, and 0.3% of women of childbearing age (15–49 y old) had BLLs ≥5μg/dL (95% CI: 0.1, 0.5). Higher BLLs were associated with older age, birthplace outside the United States, ever pregnant, a higher number of live births, lower PIR (<1.3), and not currently pregnant. In the most recent survey years (2011–2016) (Table 4), the GM BLL was 0.61μg/dL (95% CI: 0.59, 0.64) in women 15–49 years of age, and 0.7% of women of childbearing age (15–49 y old) had BLLs ≥5μg/dL (95% CI: 0.4, 1.2), which represents over 500,000 U.S. women (U.S. Census Bureau 2017). Higher BLLs were associated with older age, other race/ethnicity, ever pregnant, and not currently pregnant.

The GM BLL among U.S women of childbearing age has declined substantially from 10.37 to 0.61μg/dL over the 40-y period of this analysis (1976–2016). The reported generational decline in BLLs corresponds to successful policy initiatives over this period that reduced sources of lead in gasoline, paint, plumbing fixtures, and other consumer products (Dignam et al. 2019). In NHANES II, the first to measure BLLs, an astonishing 98.3% of U.S. women 15–49 years of age had BLLs ≥5μg/dL compared to less than 1% with BLLs ≥5μg/dL in the most recent survey period of this analysis (2011–2016). However, despite the dramatic declines in population BLLs over time, this still represents over 500,000 women of childbearing age in the United States who have exposure to lead above the CDC action level, BLLs ≥5μg/dL for pregnant women (Ettinger and Wengrovitz 2010; U.S. Census Bureau 2017). A BLL of 5μg/dL or greater in a pregnant woman flags the occurrence of prior or ongoing lead exposure that may not otherwise be recognized. The vulnerability of a developing fetus to adverse effects of lead and the possibility of preventing additional postnatal exposures justify intervention for pregnant women showing evidence of lead exposure at these levels.

Epidemiologic and experimental evidence suggest that lead is a potent reproductive and developmental toxicant (Landrigan et al. 2000; NTP 2012), but the biological mechanisms of effect are not fully understood. Maternal exposure to high levels of lead (BLL of 10–30μg/dL) have been consistently linked to an increased risk for spontaneous abortion (Hertz-Picciotto 2000). Lead also has been identified as a statistically significant predictor of maternal blood pressure (Rothenberg et al. 1999) and associated with increased risk for gestational hypertension (Karumanchi et al. 2005; Kosnett et al. 2007; Rothenberg et al. 2002). It remains uncertain at what exposure level risk begins to increase and whether or not lead-induced increases in blood pressure during pregnancy may lead to severe hypertension or preeclampsia.

Elevated BLLs among pregnant and lactating women are a particular problem due to an increased risk for exposure to developing fetuses and breastfeeding infants during key stages of development. There is no safe level of lead exposure in children (Advisory Committee on Childhood Lead Poisoning Prevention 2012). Although data are inconsistent, maternal lead exposure may increase the risk for preterm delivery, fetal growth restriction, and decreased postnatal growth (Cheng et al. 2017; González-Cossío et al. 1997; Greene and Ernhart 1991; Ingelfinger and Schnaper 2005; Sanin et al. 2001). Fetal lead exposure is predictive of adverse neurodevelopment later in life (Schnaas et al. 2006; Wasserman et al. 2000), and these effects are independent of the effects of postnatal exposure (Gomaa et al. 2002; Hu et al. 2006).

There is increasing awareness that unintended exposure to environmental contaminants, such as lead, may be adversely affecting maternal and infant health, including the ability to become pregnant, maintain a healthy pregnancy, and have a healthy baby. Women and children may be disproportionately affected by environmental risk factors that are associated with poverty and social injustice, thus making them particularly vulnerable to adverse health effects of such environmental exposures (Silbergeld and Patrick 2005). Maternal lead exposure is a serious public health concern because lead remains a widespread environmental health hazard, and current efforts at primary prevention have focused almost entirely on children. Identifying women at risk and implementing effective strategies for prevention of fetal and early infant lead exposure is an important public health priority. In fact, it may be necessary to consider prepregnancy interventions among certain high-risk groups because research suggests that screening and intervention after than the first trimester may be too late to prevent fetal neurotoxic effects (Hu et al. 2006).

In particular, lead exposure remains a concern for women in certain population subgroups at increased risk for exposure, such as foreign-born recent immigrants, and those with specific opportunity for exposure, such as individuals who may be exposed in the workplace or through the use of certain consumer products that contain lead. The U.S. Occupational Safety and Health Administration (OSHA) Lead Standard 1910.1025, established in the 1970s and updated in the 1980s, requires workers be removed from lead exposure when BLLs are equal to or greater than 50μg/dL (construction industry) or 60μg/dL (general industry), and allows workers to return to work when the BLL is below 40μg/dL (OSHA 2012). However, current guidelines for medical management of lead-exposed adults (Kosnett et al. 2007) recommend that it is advisable for pregnant women to avoid occupational or avocational exposures that would result in blood lead concentrations >5μg/dL, and, in 2015, NIOSH designated BLLs ≥5μg/dL as elevated (CDC 2018b).

Lee et al. (2005) studied determinants of blood lead in U.S. women of childbearing age using data from NHANES III (1988–1994) and found a number of factors associated with higher BLLs, including age, black or Hispanic race/ethnicity, living in the northeast region or in urban areas, lower educational level, poverty, lower hematocrit, alcohol use, cigarette smoking, and serum protoporphyrin level (Lee et al. 2005). In the more recent years, highlighted by our current analysis, several of these risk factors remain, including increased maternal age, race/ethnicity, poverty, and birth outside the United States, suggesting that lead exposure persists in certain segments of the population due to sociodemographic disparities.

Interestingly, we found that women “not currently pregnant” had higher BLLs than pregnant women in most survey cycles, which is counterintuitive given bone lead mobilization during pregnancy. However, this group of “not currently pregnant” includes women of different ages, parities, lead exposure histories, and other characteristics that may also be associated with higher BLLs (and bone lead levels). We did not conduct multivariate analyses, so it is not possible to tease out the reasons for this paradoxical finding.

The major limitation of this analysis is the incomplete ability to conduct subgroup and multivariate analyses due to small cell sizes, particularly at higher BLLs. Despite combining 40 y of survey data, the population subsample with valid blood lead test results is limited within survey periods. Therefore, several estimates, as evaluated by the RSE according to NHANES analytic guidelines, are unstable and should be reviewed with caution. Nonetheless, our analysis provides important information on long-term trends in BLLs among women of childbearing age in the United States.

The U.S. Preventive Services Task Force recently highlighted key gaps in the evidence that is insufficient to either support or recommend against blood lead screening in pregnant women and children (Cantor et al. 2019; Curry et al. 2019; Jin 2019; Spanier et al. 2019). The American College of Obstetricians and Gynecologists (ACOG), following the CDC’s 2010 guidelines (CDC 2010), recommends that providers ask about common risk factors for lead exposure and perform blood lead testing of pregnant women if risk factors are identified (ACOG 2012).

Overall, current lead exposure in U.S. women of childbearing age is generally low. The estimates presented here, based on a nationally representative sample, can be generalized to the U.S. population. However, an increasing number of reports indicate that highly exposed women do still exist, and local surveillance may identify additional risk factors for specific population subgroups. The identification of high-risk women who are pregnant or intend to become pregnant before they pass their own lead burden on to their developing fetus or breastfeeding infant is important, as there is no apparent threshold for the adverse effects of lead in children. BLLs remain higher for certain children at risk, particularly those in minority populations, from low-income families, and/or who live in older homes (CDC 2018b, 2018c). Some of these children may begin their lead exposure in utero. Continued efforts to preemptively eliminate or control sources of lead, particularly in high-risk communities, screen persons at highest risk for exposure, and provide timely interventions for those identified with elevated BLLs will help to protect future generations. The prevention of lead exposure and its adverse health effects remains an important public health issue.