Variation in newborn size according to pregnancy weight change by trimester.

BACKGROUND The timing of maternal weight change in pregnancy may be an important determinant of the newborn's size. OBJECTIVE The purpose was to identify effects of maternal weight change by trimester on newborn size, under the hypothesis that low weight gain early in pregnancy predicts proportionately smaller newborns. DESIGN Women planning to become pregnant were followed by clinic visits and questionnaires through delivery. This study includes 389 women and their singleton infants born at a gestational age of > or = 241 d. RESULTS In multiple regression analyses including a variety of potential confounders, maternal weight gain in the first and second trimesters predicted newborn weight (1-kg weight gain in the first trimester predicted a 31-g increase in newborn weight, P < 0.0007, and 1-kg weight gain in the second trimester predicted a 26-g increase in newborn weight, P < 0.007), but weight gain in the third trimester did not. Newborn ponderal index (in kg/m(3)) was predicted by weight gain in the first (1-kg weight gain predicted an added 0.21 units, P < 0.0003) and third (1-kg weight gain predicted an added 0.12 units, P < 0.03) trimesters but not in the second trimester. Newborn weight was 211 g lower (P < 0.006) and ponderal index 1.2 units lower (P < 0.02) in infants born to women who lost weight in the first trimester. CONCLUSIONS The use of measured prepregnancy weight in tests of the effect on newborn size of weight gain by time in pregnancy produces different results than does the use of recalled prepregnancy weight. Maternal weight change in the first trimester of pregnancy more strongly influences newborn size than does weight change in the second or third trimester.