Case selection and definition
We assessed blood pressure, and performed urinalysis and ultrasound, including fetal position, fetal growth, and amniotic fluid volume at each examination, screening for congenital anomalies, when needed. All maternal and ultrasound data were stored in medical records. In twin pregnancies, ultrasound was performed in the first trimester to determine chorionicity and amnionicity. That is, an intertwin membrane with the ”twin peak” or ”lambda” sign indicates dichorionic twins, and an intertwin membrane with the ”T” sign indicates monochorionic twins14. Maternal renal dysfunction was defined as a serum creatinine of above 0.8 mg/dL. Preeclampsia was defined as the occurrence of gestational hypertension (systolic blood pressure >140 mmHg or diastolic blood pressure >90 mmHg after 20 weeks of gestation), proteinuria (>300 mg on 24-hour urine collection), and/or signs of end-organ dysfunction during pregnancy15. Fetal growth restriction was defined as a birth body weight of <-1.5 SD for gestational age in Japan16.