Statistical analysis
SAS version 9.4 (SAS Institute Inc) was used for all statistical
analyses. Differences were considered statistically significant when a
2-sided P < 0.05. The general demographic characteristics of
the participating women are presented as percentages (%), and the
distributions of the characteristics were compared between women with a
history of HA-treated IUAs and matched women with no history of IUAs.
Log-binomial models using generalized estimating equations methods to
account for the paired nature of the matched sample were conducted to
estimate the relative risks (RRs) and 95% confidence intervals by
comparing the proportions of pregnancy complications, placental
abnormalities, PPH or adverse birth outcomes in exposed and matched
control pregnancies. The final model included the matched cohorts for
analyses of each pregnancy complication, placental abnormality, PPH and
adverse birth outcome, respectively, with no additional adjustment.
Women with no history of IUAs served as the reference.
In a secondary analysis, women with a history of HA-treated IUAs were
further divided into three subgroups: women who received one surgery,
two surgeries and more than three surgeries before this pregnancy. RRs
and 95% CIs for the associations between the number of surgeries and
risk of adverse obstetrical outcomes were estimated by unadjusted
log-binomial analysis using generalized estimating equations with women
with no history of IUAs serving as the reference. P-values for trend
were estimated by including the number of hysteroscopic surgeries as
continuous variables in the log-binomial models.
Based on the time intervals from the last hysteroscopic surgery to the
date of conception, all women with a history of HA-treated IUAs were
further divided into two subgroups: ≤ 3 months and > 3
months since the date of the last hysteroscopic surgery. The
associations of the time intervals with pregnancy complications,
placental abnormalities, PPH or adverse birth outcomes were further
examined. The estimated RRs and 95% CIs were reported with women with
no history of IUAs serving as the reference.
Finally, to investigate whether the associations of history of
HA-treated IUAs with risk of adverse obstetric outcomes differed
according to maternal age (< 35, ≥ 35), mode of conception
(natural conception, conceived through ART) and maternal parity
(nulliparous, multiparous), stratified analyses were conducted and the
RRs and 95 % CIs were calculated in each subgroup.