The Canadian researchers say that pre-pregnancy weight reduction could be an independent means of preventing these defects.
The team from the University of Toronto carried out a retrospective population-based study between 1994 and late 2000.
The risk of neural tube defects in babies was 1.2 per 10kg incremental rise in maternal weight. Comparing the highest with the lowest quartile of maternal weight, the adjusted risk for NTD was 2.6, they said.
Furthermore, the interaction between elevated maternal weight and the presence of folic acid flour fortification was of borderline significance, they write in the journal Obstetrics & Gynecology (105, pp261-265).
The findings add to the health risks of overweight.