Matched control pregnancies
Women with no history of IUAs were categorized as unexposed control personnel. To control for potential confounders, we matched each exposed pregnancy with four women with no history of IUAs based on propensity scores generated by maternal age and parity, mode of conception, pre-pregnancy body mass index (BMI) and prior history of abortion. In propensity score matching (PSM), the nearest neighbor matching algorithm with a caliper width of 0.2 of the standard deviation of the logit score was used. Women with a history of other hysteroscopic operations, with amenorrhea/oligomenorrhea and with intrauterine adhesions indicated by ultrasound but not received hysteroscopic surgery were further excluded from the matched controls (N=43).