Objective: To examine the relation between prepregnant weight and the risk of neural tube defects (NTDs).
Design: Data were collected from 1988 to 1994 in a case-control surveillance program of birth defects.
Setting: Study subjects were ascertained at tertiary care centers and birth hospitals in the greater metropolitan areas of Boston, Mass, and Philadelphia, Pa, and in southeastern Ontario.
Participants: Cases were 604 fetuses or infants with an NTD identified within 6 months of delivery. Controls were 1658 fetuses or infants with other major malformations identified within 6 months of delivery. For 1992 to 1994, there were 93 control infants without major malformations.
Main outcome measure: Relative risk of NTDs in infants or fetuses for different maternal weights.
Results: Relative to women who weighed 50 to 59 kg, risk of NTDs increased from 1.9 (95% confidence interval [CI], 1.2 to 2.9) for women weighing 80 to 89 kg to 4.0 (95% CI, 1.6 to 9.9) for women weighing 110 kg or more. When women were classified according to daily intake above or below the recommended level of 400 micrograms of folate, approximate threefold increases in risk were estimated for the heaviest weights in both groups. Intakes of 400 micrograms of folate or more reduced risk of NTDs by 40% among women weighing less than 70 kg, but no risk reduction was observed among heavier women.
Conclusion: The risk of NTDs increased with increasing prepregnant weight, independent of the effects of folate intake.