The woman in the case was screened with a BLL due to her country of origin, Mexico. In the US, 95% of pregnant women with elevated BLLs were born outside of the US, and the most common countries of origin are Mexico and India, followed by many other countries on all continents (19). Sources of lead exposure vary by country of origin. The most common sources of lead exposure in Mexico overall include glazed ceramics, lead-contaminated utensils, and lead-contaminated water (22). Studies of pregnant women in Mexico have found that the main predictors of BLL for this population are occupational exposure, use of lead-glazed pottery, and consumption of contaminated soil (23).
Maternal BLLs continued to uptrend in the second and third trimesters and immediately post-partum and did not follow the typical U-shaped curve. It is unclear whether the uptrend in the second trimester reflects a new exposure, maternal calcium deficiency leading to accelerated bone resorption, or missed visualization of the typical trough due to timing of levels.
Based on the current understanding of lead transfer via breast milk, the CDC guidance recommends that breastfeeding should not be initiated when maternal BLL is greater than 40 µg/dL and that serial infant BLL should be monitored closely if breastfeeding is initiated at maternal BLL 20–39 µg/dL. For this case, based on informed projections of maternal BLL and initial infant BLL, initiation of breastfeeding was initially deferred, and the mother pumped and discarded milk to maintain supply.
Without breastfeeding, the infant’s BLL decreased from birth to 1 month. At 1 month, maternal BLL was 30 µg/dL and infant BLL was 22 µg/dL, and breastfeeding was initiated with close monitoring. Two weeks later, the infant’s BLL increased to 28 µg/dL, and breastfeeding was paused for two weeks.
The increase in infant BLL after initiation of breastfeeding and the subsequent decrease in infant BLL after a 2-week break from breastfeeding suggests that the source of lead was transfer via breast milk. The CDC models are based on data between birth to 1 month, but if this model is applied to this slightly later timeframe, a maternal BLL of 30 µg/dL predicts that the infant BLL would increase by 3.7 µg/dL in a month. In our case, the infant BLL increased by 6 µg/dL in two weeks, a rate that is more than 3-fold greater than predicted. The degree of BLL increase with initiation of breastfeeding prompts further consideration.
The timing of the maternal BLL peak may help explain the degree of increase in infant BLL. Though the recorded peak in maternal BLL was 30 µg/dL at 1 month postpartum, the postpartum/lactational peak may have occurred after this measurement, contributing to higher infant BLL than expected based on the CDC model.
Maternal calcium deficiency at the time of breastfeeding initiation should also be considered, though the mother’s reported diet and supplementation reflected adequate calcium intake. Her lab results also reflected adequate calcium intake, with a normal total calcium, phosphorus, and PTH despite biochemical vitamin D deficiency.
The increase may also represent an environmental source, such as staying at a family member’s home, but no such history was elicited in this case. It may also represent the feeding of breastmilk expressed in the first month postpartum, but this milk was reported to have been discarded.
Lab error and sample variability can always be considered. However, even assuming maximum acceptable variability in each of the measurements, the infant’s BLL after initiation of breastfeeding would still represent a greater than 2 µg/dL increase in BLL, the measurement error range in our lab. This would raise concern about ongoing exposure and ingestion, presumably from the breastmilk in this case.
Another question to consider is the variability of lead content in breast milk. At the same BLL, what is the variation in breast milk lead content? Many studies have attempted to quantify a milk/blood ratio or milk/plasma ratio for lead (24)(25), which is widely variable, likely due to technical challenges in breast milk lead measurement, differences in lead content in colostrum versus mature milk, environmental contamination, and perhaps differences in the rate of transfer to breast milk. In 2014, Ettinger and colleagues, whose earlier data provided the basis for the linear CDC projections of infant BLL changes based on maternal BLL, found that milk/plasma ratio was higher at lower plasma levels, suggesting a paradoxically more efficient transfer of lead into breast milk at lower plasma levels (26).
After the 2-week break from breastfeeding at 2 months old, the infant’s lead level decreased to 25 µg/dL and breastfeeding was resumed until discontinuation at 6 months old. Throughout this period, the infant’s BLL declined appropriately (also mirroring mother’s BLL) with two exceptions. First, between 2 months and 4 months, there is a slower decline in the infant’s BLL as compared to the maternal BLL, perhaps reflecting ongoing exposure to lead via breast milk. Second, the infant’s BLL increased by 2 µg/dL between 6 and 7 months. Since this occurred after discontinuation of breastfeeding, it is more likely to represent a different source of exposure. Another possibility is inter-lab variability, as these consecutive tests were conducted at two different labs.
Current models provide an excellent foundation for appropriate decision-making about breastfeeding in the case of maternal lead exposure, but ongoing research is needed to better understand the complexities of lead transfer in lactation. This case demonstrates that close monitoring and strategic short term pauses in breastfeeding can help alleviate the burden of lead transfer in the breastfeeding dyad and facilitate the ongoing breastfeeding relationship.