Maternal weight gain and preterm delivery: Differential effects by body mass index

We examined associations between weight gain (kg) per week of pregnancy and net weight gain per week of pregnancy (weight gain - birth weight/weeks of gestation at delivery) and preterm delivery in a population of 266,172 low -income women. Risk of preterm delivery was lowest among women with interme diate weight gain (0.35 to <0.46 kg/week) and net weight gain (0.27 to <0.3 7 kg/week). Both lower and higher weight gains and net weight gains per wee k were associated with an increased risk for preterm delivery. Associations , however, were not uniform across body mass index categories. Compared wit h women gaining 0.35 to <0.46 kg/week, preterm risk differences (95% confid ence limits) for women gaining <0.10 kg/week were +9.5% (+6.5, +12.4) for u nderweight women, +6.7% (+5.6, +7.9) for average-weight women, +3.5% (+2.0, +4.9) for overweight women, and +0.4% (-0.4, +1.2) for obese women. The op posite pattern was observed with high weight gain. Preterm risk differences for weight gains >0.65 kg/week ranged from +0.8% (-0.7, +2.1) for underwei ght women, to +2.5% (+1.3, +3.9) for obese women. We also evaluated weight gain per week in the latter part of pregnancy (from week 14 to delivery). T he same basic patterns were observed; however, variation in the association s across body mass index groups was not as marked.