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).