aspirin,tablet,100/150mg per day,(BMI<30 100mg/d，BMI≥30 150mg/d)，from pregnancy weeks<16 to 35 weeks or the day of delivery.

with no intervention

With the result of risk assessment for high risk and the risk of pregnant women, with each research center stratified random aspirin drug intervention, for each center screening high-risk and accord with the standard drug intervention study phase into eliminate pregnant women by using stochastic indicator 1:1 randomly into low-dose aspirin to prevent group and observation group, for each center in screening for risk and comply with the drug intervention study phase into the exclusion standard 1:1 by random indicator method pregnant women randomly into low-dose aspirin to prevent group and observation group.

Hypertensive onset after 20 weeks of gestation is accompanied by proteinuria,or with thrombocytopenia, impaired liver function, renal insufficiency,pulmonary edema,headache without other explanation and so on.

gestational hypertension, preeclampsia, chronic hypertension with preeclampsia, eclampsia, HELLP syndrome

After 20 weeks of pregnancy, the placenta in its normal position is completely or partially removed from the uterine wall before delivery of the fetus

The birth weight of the fetus is below two standard deviations of the average weight for the same gestational age or below the 10th percentile of normal weight for the same age

Within 24 hours after the delivery of the fetus, the bleeding volume of vaginal delivery ≥500ml, cesarean delivery ≥1000ml

gestational week, vaginal delivery and cesarean section

macrosomia, normal weight, low birth weight

≥8；4-7,；≤3