Legacy of Lead.

Babies at Risk Decades after Mothers Exposed

Fetal lead exposure is associated with delayed embryonic development of several organ systems and cognitive deficiencies in early childhood. Infants and children are more susceptible to the metal's effects than adults, and women exposed to high levels of lead decades ago may still pass the toxic metal to their unborn children. In a bid to better estimate fetal lead exposure from the mother, Hung-Yi Chuang of the Harvard School of Public Health and colleagues studied lead transfer in utero in Mexico City, where residents are exposed to large amounts of the metal through sources such as lead-based gasoline and lead glaze commonly used in cooking vessels [EHP 109:527-532]. They found that estimating maternal plasma lead concentrations using structural equation modeling yielded a more accurate estimate of fetal lead exposure than the traditional method of measuring maternal whole blood lead concentrations.

Lead in the circulatory system is held mostly in the red blood cells; less than 1% is in the plasma. But only plasma lead can cross the placenta to the fetus. Measuring plasma lead concentrations is costly and difficult, so researchers had previously estimated concentrations based on the lead concentrations in whole blood. The two may not be in equilibrium, though, particularly during pregnancy, when stored lead is released into plasma from bone.

Structural equation modeling, a technique most often used in the social sciences, allowed the researchers to account for interrelationship among variables, unlike the more traditional multivariate regression technique. This was important because plasma lead concentrations are affected by several variables, including use of lead-glazed ceramics, years spent in a lead-polluted area, and bone lead concentrations.

The researchers studied 615 pregnant women receiving prenatal care at one of three hospitals in Mexico City in 1994 and 1995. They gathered maternal and umbilical cord blood samples within 12 hours of each infant's delivery, then measured maternal bone lead within one month of delivery. The researchers also asked their subjects several lifestyle questions to assess their recent and lifetime exposure to lead and obtained data on regional air lead concentrations during the two months before each delivery. By examining all these factors, the researchers developed what they believe is a highly accurate estimate of fetal lead exposure.

The researchers found that maternal bone lead is a significant source of fetal exposure. Bone serves as the main depository of lead in adults. Because it has a half-life of 7-20 years in bone, stored lead could stem from exposure many years earlier. Pregnancy and lactation cause large amounts of minerals to release from bone in order to help build the fetus's skeletal system. Because lead is a "calcium imposter," substituting for calcium in the bone-mineral matrix, it's released along with calcium even though it isn't needed by the fetus--and along with calcium, it's incorporated into the fetus's developing bones and tissue. Infants also require calcium in breast milk to support their growth, so lead is released from the mother's bone to breast milk along with calcium and other minerals.

Although the researchers limited their study to Mexico City, they believe babies in the United States and other industrialized countries face the same risk from mothers raised in an era when lead in the atmosphere was not as well controlled as it is now. Their study suggests that blood testing combined with X-ray exams and a lifestyle questionnaire could help predict the risk of fetal lead exposure before a woman becomes pregnant. (Nonessential X rays are considered too risky to administer during pregnancy.) Women at high risk then could be treated to minimize lead transfer to their fetuses. Animal studies have shown that calcium supplements minimize mineral loss from bone during pregnancy. Researchers involved in this study are now testing how calcium supplements affect the transfer of lead from pregnant Mexico City women to their fetuses.