['To clarify whether mothers with gestational weight loss (GWL) were likely to have adverse effects on the placenta.', 'Subjects who delivered viable singleton infants after 24 weeks of gestation were enrolled. A retrospective analysis to evaluate cases of GWL in association with the findings of the placenta and amniotic membrane after delivery was conducted. After consideration of confounders, a case-control study with matched pairs (1:2) was performed.', 'Of all subjects (5551 cases), 83 cases (1.5%) with GWL were found. Since the pre-pregnancy maternal body mass index (BMI) was significantly higher in cases, 166 controls with a matched BMI were selected. The neonatal birth weights, placental weights and the umbilical cord length in cases were significantly smaller than in controls (p < 0.05). Preterm delivery and small for gestational age (SGA) infants were more frequently observed in cases compared with controls [odds ratio (OR) 6.3; 95% confidence interval (CI) 3.3, 12.1, OR 4.3; 95% CI 1.9, 9.9]. pPROM were observed in 10.8% of the cases and 1.8% of the control (OR 6.6; 95% CI 1.7, 25.1). However, the frequencies of chorioamnionitis and the cervical length at second trimester were not different between the two groups.'] Does gestational weight loss have adverse effects on placental development?