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Objective Adequate maternal weight gain in twin pregnancies is associated with improved outcomes such as increased fetal growth and decreased incidence of preterm birth. However, it remains unclear when gestational weight gain has the greatest influence on pregnancy outcomes. Our objective was to identify at which time in a twin pregnancy does inadequate maternal weight gain have the greatest association with adverse pregnancy outcomes. Study Design This is a retrospective cohort study of women with twin pregnancies and normal prepregnancy body mass index (BMI, 18.5–24.9 kg/m2) who delivered at ≥24 weeks' gestation by a single maternal–fetal medicine practice between 2005 and 2017. Baseline characteristics and pregnancy outcomes were compared between women with and without adequate average gestational weight gain (weight gain per week based on the 2009 Institute of Medicine recommendations). This analysis was performed for weight gain over the entire pregnancy, as well as from 0 to 16, 16 to 24, and 24 weeks to delivery. Multivariable regression analysis was performed to control for potential confounding variables. Results A total of 609 women with twin pregnancies and normal prepregnancy BMI were included, of whom 386 (63.4%) had adequate average gestational weight gain over the entire pregnancy and 223 (36.6%) did not. Inadequate average gestational weight gain between 0 and 16 weeks' gestation was associated with a higher incidence of birthweight less than the 10th percentile for gestational age (adjusted odds ratio [aOR]: 1.67; 95% confidence interval [CI]: 1.11–2.51) and less than the 5th percentile for gestational age (aOR: 2.10; 95% CI: 1.29–3.40). Inadequate gestational weight gain between 16 and 24 weeks was associated with lower birthweight of the larger twin (β: –0.09; p = 0.04). Inadequate weight gain from 24 weeks to delivery was associated with spontaneous preterm birth <37 weeks' gestation (aOR: 1.67; 95% CI: 1.13–2.47), <34 weeks' gestation (aOR: 4.32; 95% CI: 2.45–7.63), <32 weeks' gestation (aOR: 9.07; 95% CI: 3.66–22.48), and a lower incidence of preeclampsia (aOR: 0.31; 95% CI: 0.16–0.63). Conclusion In twin pregnancies, gestational weight gain between 0 and 16 weeks as well as between 16 and 24 weeks is most associated with fetal growth, whereas gestational weight gain after 24 weeks is most associated with preterm birth. This may help elucidate the mechanism of action of the impact of gestational weight gain in twin pregnancies.

In patients undergoing sonoEFW within 2 weeks of delivery, sonoEFWs ≥4000 g are significantly more likely to overestimate than underestimate the true BW. Obstetricians should be cautious about intervening based on sonoEFW alone, given the high risk that this value is an overestimation of the true weight.