Objectives Although occupational and environmental exposures to lead have been dramatically reduced in recent decades, adverse pregnancy outcomes have been observed at ‘acceptable’ levels of blood lead concentrations (≤10 μg/dl).
Methodology Blood samples were collected from 348 singleton pregnant women, aged 16–35 years, during the first trimester of pregnancy (8–12 weeks) for lead measurement by inductively coupled plasma–mass spectrometry. Subjects were followed up and divided into two groups (preterm and full-term deliveries) according to duration of gestation.
Results The average (range) and geometric means of blood lead levels were 3.8 (1.0–20.5) and 3.5 μg/dl, respectively. Blood lead level was significantly (p<0.05) higher in mothers who delivered preterm babies than in those who delivered full-term babies (mean±SD: 4.46±1.86 and 3.43±1.22 μg/dl, respectively). Logistic regression analysis demonstrated that a 1 unit increase in blood lead levels led to an increased risk of preterm birth (OR 1.41, 95% CI 1.08 to 1.84).
Conclusion Adverse pregnancy outcomes may occur at blood lead concentrations below the current acceptable level.
- preterm birth
- gestational age
- female reproductive effects and adverse pregnancy outcomes
Statistics from Altmetric.com
- preterm birth
- gestational age
- female reproductive effects and adverse pregnancy outcomes
What this paper adds
Blood lead at ‘acceptable’ levels can cause adverse pregnancy outcomes.
Despite previous studies, the toxic effects of low levels of lead on preterm delivery are still controversial.
The current study showed that average±SD blood lead levels of <4±2 μg/dl pose a risk of preterm birth.
As blood lead measured during the first trimester of gestation may represent pre-pregnancy levels, pregnant women and women of childbearing age should avoid lead exposure even if their blood lead concentrations are <10 μg/dl.
Lead is one of the oldest known metals, and occupational/environmental exposures to lead are widely studied. The toxic effects of lead are mediated by haemopoietic, nervous system, reproductive, blood pressure and urinary tract functions.1 2 Recently, researchers turned their attention to clinical and subclinical disorders associated with low concentrations of lead after levels of exposure declined dramatically in past decades.2–4 Several major adverse health outcomes have been reported at blood levels lower than the supposedly ‘acceptable’ concentration (≤10 μg/dl) adopted by the Centers for Disease Control and Prevention in 1991 and WHO in 1995.5–7
During pregnancy, long-term fetal exposure to lead via the mother causes lead accumulation in fetal tissues and may lead to irreversible damage.8 Previous studies have shown that maternal blood lead levels ≤10 μg/dl may cause complications during pregnancy, including increased risk of pregnancy hypertension, reduced length of gestation, miscarriage, spontaneous abortion and preterm delivery.9–12 Recently, adverse pregnancy outcomes have been reported even at mean blood lead levels of <5 μg/dl.5 13 In addition to exogenous lead exposure, lead in the bone can be a potential endogenous source for increasing blood lead concentrations in previously exposed women.14 15 This discharged lead can freely cross the placenta and affect the fetus.16
Preterm labour can cause perinatal morbidity and mortality and long-term handicap in surviving infants. It may be induced by many factors, including gestational hypertension, multiple pregnancy, intrauterine growth restriction, maternal stress, a heavy physical work and smoking.17–19 Environmental factors combined with inherent genetic susceptibility may contribute to an increased risk of preterm labour in some women.20 The incidence of preterm birth in most countries is less than 10% of live births, but, in both developed and developing countries, this rate has tended to rise in recent decades.21 22 In Iran the frequencies of reported preterm delivery ranges from 3% to 39%.23 24
Previous reports on the effects of low levels of lead on pregnancy outcome1 25 have investigated the pathways of exposure, duration of exposure, and individual differences among subjects. This study aims to clarify the effects of low levels of blood lead measured in early pregnancy and at preterm delivery in apparently healthy women and is part of a longitudinal study project which began in October 2006 in three teaching hospitals in Tehran, Iran.
Of 1238 pregnant women referred to hospital for prenatal care during the study recruitment period, 667 were disqualified because they were not in the first trimester of pregnancy and 136 because of obesity (body mass index (BMI) >30), cigarette smoking, multiparity (parity>2) and/or chronic conditions such as heart disease, hypertension, diabetes, cancer or renal failure. Thirty two women refused to enter the study, mainly because the procedure required the taking of blood samples. From among the remaining eligible women, singleton pregnant women who were referred for continuous prenatal care from early pregnancy (8–12 weeks) to delivery were recruited on a volunteer basis (35 unmatched subjects were excluded). Each subject's complete medical and prenatal records were reviewed and general medical laboratory tests including complete blood count, urinalysis, blood sugar, protein A1C, BUN/creatinine, and liver function (SGOT, SGPT and alkaline phosphatase) were carried out. The Ethics Committee under the Vice-chancellor for Research, Tehran University of Medical Sciences, approved the study design and procedure. All subjects were informed verbally of the purpose and procedures of the study.
Subjects underwent utero-abdominal sonography to assess gestational age, evaluate the fetus and check for multiple pregnancy, ectopic pregnancy and placenta displacement. This resulted in four women being excluded. A structured questionnaire was then filled out for each subject in a face-to-face interview to determine socioeconomic background, anthropometric variables, habits and medical/reproductive history. The subjects were followed up and classified as preterm or full-term deliveries according to the duration of gestation; preterm delivery was defined as a delivery after 20 and before of 37 weeks of gestation. The mother's weight (kg) at the time of recruitment was measured using a standard scale. BMI was calculated as weight (kg) divided by height squared (m2).
Collection and analysis of blood samples
Blood samples were collected from the cubital vein using lead-free vacuum tubes (Venoject VP-H070K, Terumo, Tokyo, Japan) and stored at −70°C until transfer to Japan for blood lead measurement. Blood samples (in 1.0 ml volumes) were accurately weighed and transferred to a perfluoroalkoxy Teflon bottle, 4 ml of concentrated nitric acid (Ultrapure Grade, Tama Chemicals, Kawasaki, Japan) was added and the mixture left overnight. The sample mixture was then digested with 0.8 ml hydrogen peroxide and 0.8 ml perchloric acid (Ultrapure Grade, Tama Chemicals) in a microwave oven (MLS-1200 MEGA, Milestone, Sorisole, Italy) in five steps with power set at 250, 0, 400, 650 and 250 W for 6, 1, 6, 6 and 6 min, respectively; the volume of the digested sample was then adjusted to 10.0 ml with ultrapure water. After dilution with 0.5% nitric acid, concentrations of lead were measured by inductively coupled plasma–mass spectrometry (ICP-MS, Eran6000, PerkinElmer, Waltham, Massachusetts, USA) at m/z 208 using the multi-element standard solution XSTC-13 (SPEX CertiPrep, Metuchen, New Jersey, USA). Measurements were repeated three times. For instrument calibration throughout the measurements, at least 10% of the analyses were external standard, and 5% were blank (pure water).
Student t tests, χ2 tests and Fisher's exact tests were used to compare the characteristics of the preterm and full-term groups. Pearson's correlation coefficient was calculated to assess associations between gestational age and blood lead concentrations. To reduce the influence of outliers and normalise residual distribution, we used the natural logarithm of blood lead in the statistical analysis. A logistic regression analysis was performed, where preterm labour (=1) or not (=0) was used as the dependent variable; blood lead levels, age, sex of the newborn, education, passive smoking, pregnancy weight gain, parity, haematocrit and type of delivery were defined as independent variables. SPSS software was used for the analysis.
Sixteen of the 364 pregnant women recruited into the survey delivered their babies before the 20th week of gestation. This left 348 women, mean age 25 (range 16–35) years, of whom 44 had preterm babies (12%). The blood lead measurement results of 12 (3%) women were unavailable. The mothers' mean±SD blood lead level was 3.8±2.0 μg/dl (range 1.0–20.5) with a geometric mean of 3.5 μg/dl. Due to outlier values of blood lead all analyses were also carried out after exclusion of these subjects (n=6); the main findings were not significantly changed.
Blood lead concentrations were significantly higher in the preterm mothers than in the full-term mothers (table 1). Logistic regression analysis demonstrated a significant association between increased blood lead concentrations and risk of preterm labour (OR 1.41, 95% CI 1.08 to 1.84) after adjustment for potential risk factors for preterm birth. Negative correlations between the subjects' blood lead and gestational age are shown in figure 1.
This study found that mothers who delivered preterm babies had significantly higher blood lead levels than mothers who delivered full-term babies. Logistic regression analysis showed an increasing risk of preterm birth with increasing blood lead concentration. A negative correlation between blood lead levels and gestational age was also found. Therefore, the current results suggest that raised blood lead concentrations, even at ‘acceptable’ levels, could be a risk factor for preterm delivery. In agreement with the present study, Fahim et al26 in Missouri have reported higher blood lead levels in mothers with preterm deliveries than in women who delivered term babies in Columbia (26.0 vs 13.1 μg/dl, respectively) and Rolla (29.1 vs 14.3 μg/dl, respectively). Similarly, an increased risk of preterm birth has been observed at a blood lead concentration of ≥10 μg/dl compared with lower levels (OR 3.2).27 In addition, raised lead levels in the umbilical cord, membrane tissue and blood have been reported in mothers who delivered preterm babies.28 Moreover, the relationship between lead and preterm delivery seems to be dose dependent.29 However, a study on very low maternal blood lead levels (average (SE) of 1.2 (0.03) μg/dl) failed to demonstrate a significant association between lead and risk of preterm delivery.5 A slight increase in blood lead levels might present a risk for preterm delivery; further studies are necessary to identify the ‘safe’ threshold level.
Significant negative relationships between blood lead concentrations and gestational age were obtained when Pearson's correlation test was performed on the logarithmic value of blood lead levels of <10 μg/dl (figure 1). ; the correlation was also significant for the original value of blood lead at <10 μg/dl (part C in figure 1) (r= −0.15 and p=0.011).Jelliffe-Pawlowski et al27 reported that at blood lead levels >10 μg/dl, an increase of 1 μg/dl in blood lead concentration decreased length of gestation by 0.3 days. Other studies, however, have reported that elevated blood lead levels increased the duration of pregnancy.6 11 Unlike the present study, these earlier reports usually collected blood samples at the time of delivery. Due to increased bone lead mobilisation into the blood in pregnancy, blood lead concentrations can rise gradually until delivery.30 Therefore, the time of blood sampling could be an important factor: mothers with who delivery at or near term are exposed for a longer period to bone-released lead than mothers who deliver before term.
Caesarean section was more common than vaginal delivery in the preterm than in the full-term mothers (43% and 23%, respectively) (table 1). As a caesarean section is often performed several days before uterine contractions would naturally begin, it can decrease the duration of gestation. Pearson's correlation coefficient analysis was therefore performed to identify the correlation between gestational age and both blood lead levels and the logarithmic value of blood lead in mothers who underwent vaginal delivery (r=−0.22 and r=−0.19, and p=0.016 and p=0.036, respectively). Logistic regression analysis also demonstrated a significantly higher risk of preterm birth in response to elevated blood lead in the presence of caesarean section. Thus it can be concluded in the present study that any shortening of pregnancy due to blood lead is independent of the type of delivery.
The pathophysiology and molecular mechanisms involved in the impact of lead levels on pregnancy outcomes, such as preterm labour, are not yet clearly understood, but there are some probable pathways. Balanced reproductive hormones are important during pregnancy, from the implantation of the blastocyst to the onset of parturition and increasing uterine contractility. For example, the pituitary gland of the fetus increases oxytocin secretion, which might play a role in birth.31 Experimental animal studies have shown that lead, in different concentrations and following different durations of exposure, can disruption signals within and between the hypothalamus and pituitary gland and induce hypothalamic–pituitary axis imbalances.32–34 Among other hormonal changes, the oestrogen-to-progesterone ratio is increased substantially towards the end of pregnancy and may be partly responsible for the increased contractility of the uterus.31 Several studies have reported that oestrogen and progesterone secretion can be affected by lead via increases in luteinising hormone32 35 and follicle stimulation hormone.36 Thus lead-induced reproductive hormone disruption might be one of the pathways for early delivery following preterm labour.
Second, premature delivery might be induced by increased uterine muscle spasms. Uterine smooth muscle cells generally contract when calcium enters from the extra-cellular fluid following action potentials or other stimuli.31 Lead might shorten the duration of gestation by inducing uterine contractility through calcium-mediated control37 and stimulation/inhibition of protein kinase in smooth muscle cells.38 In addition, lead-induced smooth muscle cell contraction may also occur independently of extracellular and intracellular calcium stores.39 Thus, lead-induced uterine smooth muscle cell contractility could be an alternative molecular pathway and influence premature delivery in subjects exposed to lead. Lead might also facilitate preterm birth by other mechanisms, such as inducing the production of reactive oxygen species or effecting nervous system inhibition/stimulation.
As blood lead concentration and blood volume significantly increase during the second half of pregnancy,31 40 the current study collected blood samples during the first trimester when lead levels are similar to the mother's pre-pregnancy lead concentrations. Pregnant women and women of childbearing age intending to become pregnant should be advised to avoid lead exposure, even if their blood lead levels are as low as 10 μg/dl.
The present results support previous findings of adverse pregnancy outcomes at blood lead levels lower than the currently ‘acceptable’ levels, although this study may be the first to attribute preterm deliveries to low levels of blood lead (mean <5 μg/dl). On the other hand, it is possible, that in the present study, blood lead levels may serve as a proxy for some other unmeasured factor responsible for the increased risk of premature delivery (such as an underlying inflammatory state causing premature delivery as well as increased bone resumption resulting in increased blood lead). As few studies have focussed on associations between human blood lead concentrations and preterm birth, the results of the current study should be considered by future studies.
The authors thank Mr Christopher Holmes, Office of International Academic Affairs, Faculty of Medicine, the University of Tokyo, for revision of the English manuscript.
Funding This study was supported by the Japanese National Institute of Occupational Safety and Health, a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science, and Tehran University of Medical Sciences.
Competing interests None.
Ethics approval This study was conducted with the approval of the Ethics Committee under the Vice-chancellor for Research, Tehran University of Medical Sciences.
Provenance and peer review Not commissioned; externally peer reviewed.
If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.