['This study evaluates the impact of an average change in body mass index (BMI) during the first 16-25 weeks on outcomes of triplet pregnancies.', "In this retrospective observational study we evaluated a cohort of triplets born to 1235 nulliparas and 705 multiparas. The difference between the pregravid body mass index (BMI) and that obtained upon admission at 16-25 weeks' gestation was averaged to obtain the weekly change in BMI, defined as slow, typical, or fast by values<1SD, +/-1SD, and >1SD from the mean average weekly change in BMI. We compared gestational age and birth weight parameters in these three subgroups and by parity.", 'The average weekly BMI-adjusted weight gain was 0.18+/-0.08 and 0.17+/-0.08 kg/m2/week for nulliparas and multiparas, respectively. In both parity groups, differences were noted between slow and typical and between slow and fast, but not between typical and fast weight gain. Nulliparas with slow weight gain had a significantly higher incidence of infants weighing<1000 g (OR 2.0-2.5), 1000-1500 g (OR 1.4 compared with fast weight gain), and included 60-100% more sets with at least 1 SGA infant. In multiparas, there was no effect on gestational age, but otherwise, a similar trend for birth weight parameters was found.'] Is slow change in body mass index during early triplet pregnancy associated with decreased birth weight?