Data sources and study population

This study is a sub-project of Mother and Child Cohort Study in liuyang, which was designed to study pregnancy complications and pregnancy outcomes. Liuyang is a ‘county-level city’ with 4 city districts and 33 towns. Stratified random sampling was used to select the study participants. According to the urban-rural ratio, and population density and fertility levels, 2 districts and 11 towns were randomly selected in Liuyang city. All nulliparous women who accepted their first prenatal visit less than 12 weeks gestation to local maternity care unit from June 2013 to November 2014 were recruited in this sub-project if they (1) lived in those selected 13 sites, (2) had regular sex partners, (3) were not less than 20 years old, (4) had a history of first-trimester IA or no IA, and (5) were willing to give informed consent. Women were excluded if they (1) had both previous IA and previous spontaneous abortion, (2) had a multiple pregnancy (3) didn’t conceive naturally, and (4) had a uterine malformation. At the last follow-up in November 2015, finally 5191 primipara were included in this study.