In summary, we found both low and high rates of GWG during the second and third trimesters to be positively associated with preterm delivery, although the U-shaped relationship may differ by reason for preterm delivery. In our study, the association of high GWG with medically indicated preterm delivery was driven by the high weight gain among women with preeclampsia. Well powered, prospective studies starting very early in pregnancy and collecting detailed information on maternal diet and body composition, fetus, placenta and pregnancy complications are needed to determine when and whether weight gain is influential on preterm delivery or whether associations are related to common antecedents.