Outcome Measures
The primary outcome was the incidence of PE and the secondary outcomes included the gestation of birth (allowing assessment of preterm birth) and birth weight (allowing assessment of the fetus is small for gestational age). The clinical safety of aspirin was evaluated by the occurrence of antenatal or postpartum hemorrhage. PE was defined by evaluated blood pressure (blood pressure≥140/90mmHg associated with proteinuria (>300 mg/day) or blood pressure≥160/110mmHg alone) after 20 weeks of gestational age.13, 15 Blood pressure was measured by a mercury sphygmomanometer and urine samples were collected and tested by the clinical laboratory in each hospital. Once diagnosed, patients would be treated as standard treatment guidelines. The fetus being small for gestational age (SGA) was defined as the birth weight of either twin was below the 10th percentile of the average in the same gestational age. PTB was regarded as delivering before 34 weeks 0 days for twin pregnancies.16, 17 When calculating the amount of bleeding, intraoperative blood loss was recorded by the container of suction apparatus, while postoperative bleeding volume was counted by the weighing method. Cesarean postpartum hemorrhage was defined as postoperative bleeding volume more than 1000ml in 24h. These diagnostic criteria were unified in two centers before the cohort study was initiated.