285 DISCUSSION: In our study, prior pregnancy outcomes of 7,925 women with a histopathological diagnosis of adenomyosis were compared to 4,615,803 women of the general Dutch population without adenomyosis. When corrected for common confounders, women with histopathological adenomyosis had an increased prevalence of HDP and SGA infants. Furthermore, women with adenomyosis more often had an emergency caesarean delivery, failure to progress and placental retention. There was no significantly increased risk for HELLP, eclampsia, placental abruption, operative vaginal delivery, or need for oxytocin stimulation. No previous studies have investigated progress of labour in women with histologically-proven adenomyosis. Adenomyosis is thought to affect uterine contractile function due to the associated disruption of the junctional zone, leading to symptoms such as dysmenorrhea and infertility (77,289) . Uterine contractile function is arguably most well-known in the context of the onset and progress of labour, where common obstetric complications may be associated with ineffectual contractions. It can therefore be hypothesized that adenomyosis in pregnancy leads to a higher risk of these obstetric outcomes. Aberrant (specifically, premature) uterine contractile function during pregnancy can also be related to premature birth (PTB). Past reported OR’s for PTB in women with adenomyosis have ranged from 1.96 (102) to as high as 24.53 (103). Strikingly, our study cannot confirm this finding, with a lower risk of preterm birth in the adenomyosis group as compared to the general population (aOR 0.76 (95% CI 0.69-0.84). The adenomyosis group did however show an increased risk for threatened preterm birth including PPROM and cervical insufficiency. Potentially, this discrepancy lies in differences in (past) Dutch management protocols of PPROM and premature labour compared to previously published studies, leading to a later gestational age at birth. Another potential explanation for the difference in the results, is that most existing studies included mainly assisted reproductive technology (ART) patients in their populations (290,291), in contrast to our study. This may be a confounding factor leading to higher incidence of PTB in previous studies (although most studies did correct for mode of conception). Hypertensive disorders of pregnancy are thought to arise from impaired implantation and placentation due to defective spiral artery development and
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