Pre- and postnatal lead effect on head circumference: A case for critical periods

We examined the association of maternal prenatal [range of median blood lea d level 7.5-9.0 mu g/dl (0.36-0.43 mu mol/l) during pregnancy] and child po stnatal blood lead level [range of median blood lead level from birth to 48 months 7.0-10.0 mu g/dl (0.33-0.48 mu mol/l)] with head circumference in f rom 119 to 199 children from the Mexico City Prospective Lead Study. We use d repeated multiple regression modeling with a standard set of control vari ables, entering blood lead level last. Using Bonferroni-corrected probabili ty values to control for inflation of Type I error due to multiple testing at each age, we found significant negative associations (p < 0.05, two-tail ed) between 6-month head circumference and 36-week maternal blood lead leve l, and 36-month head circumference and 12-month blood lead level. Over the 25-75% interquartile range of measured blood lead, head circumference decre ased around 0.4 cm. Over the 1-35 mu g/dl (0.05-1.68 mu mol/l) range of mat ernal blood lead at 36 weeks, the estimated reduction in 6-month head circu mference was 1.9 cm (95% CI = 0.9-3.0 cm). These results suggest that child ren are more vulnerable to certain effects of lead exposure at specific age ranges, and that the effect of lead on head circumference only becomes evi dent for brief periods in the first 4 years of life. We discuss various art ifacts as well as possible mechanisms by which lead might have produced the observed pattern of results. We suggest that higher lead exposure prevalen t several decades ago might have subtly influenced published normative huma n growth data. (C) 1999 Elsevier Science Inc. All rights reserved.