We do not know when oxytocin receptor (OXTR) expression increases during gestation above non-pregnant levels or when the myometrium becomes responsive to oxytocin. Oxytocin is the most effective pharmacologic prophylaxis of hemorrhage in term deliveries, but utility in second trimester abortion is unknown. Prior research has indicated there is minimal oxytocin receptor expression in early second trimester pregnancy, suggesting that expression either gradually increases or appears at some point during the second trimester. This lack of knowledge has led to inconsistent practice; some physicians use exogenous oxytocin at gestational ages when it may be ineffective and others forego oxytocin when there may be a benefit.

In this descriptive study, we will compare relative myometrial oxytocin receptor mRNA expression using quantitative PCR and calcium binding assays of cultured myometrial cells in women undergoing surgical abortion in the second trimester (between 13-26 weeks), to non-pregnant women and to women following first-trimester abortion and term pregnancy. The primary objective of this study is to determine when oxytocin receptor expression in the myometrium increases during gestation to the near 12-fold relative expression that has previously been documented at term compared with early gestation. While these data do not reflect myometrial function, they would provide important insight into our understanding of when oxytocin can be expected to function in the management of post-abortion hemorrhage. We will obtain myometrial tissue samples by core needle biopsy under ultrasound guidance in anesthetized women immediately following surgical abortion and from controls. The outcomes of this study will further inform provider management of post-abortion hemorrhage and provide baseline data for future interventional studies about receptor function.