Risk of spontaneous preterm birth and preconceptional folate supplementation.

Incidence expressed per 1,000 women per week in a gestational age period (20�C28, 28�C32, or 32�C37 wk). Risk estimates are calculated in comparison to the group without preconceptional folate supplementation. HR adjusted for maternal age, body mass index, race and ethnicity, educational level, marital status, smoking, parity and history of prior preterm birth and recruitment center. Number needed to treat (NNT) for folate supplementation ��1 y is 435 women to prevent one case of spontaneous preterm birth between 20 and 28 wk, and 500 women to prevent one spontaneous preterm birth between 28 and 32 wk.