Postpartum hemorrhage (PPH) is the loss of more than 500ml of blood within 24 hours after delivery. It is a major cause of maternal morbidity and mortality across the globe.

Oxytocin is a naturally-occurring hormone that causes the uterus to contract, thereby causing labor. A synthetic form of oxytocin is used in obstetric medicine in the same way. It causes the uterus to contract by acting at the oxytocin receptor (OTR). It is used for both the prevention and the treatment of PPH. By causing the uterus to contract, it constricts the blood vessels within it, thus reducing bleeding.

Oxytocin is also used to augment labor in women with slow labor progression. Desensitization of the OTR after prolonged exposure to oxytocin occurs, leading to reduced contractions of the uterus with the same doses of oxytocin. This has been demonstrated in previous studies done by the investigators. The resultant need for a higher oxytocin dose to cause adequate uterine contraction has also been demonstrated in laboring women having received oxytocin for labor augmentation.

Magnesium sulphate (MgSO4) is widely used within obstetric medicine. It is used for seizure prevention and treatment in preeclampsia and eclampsia, and is used for fetal neuroprotection in preterm labor, to reduce the risk of cerebral palsy. It is well-known to have a relaxant effect on uterine muscle, and as such, has been used as a tocolytic agent in preterm labour to prevent premature contractions and premature delivery. The effect of MgSO4 on contractions in oxytocin pre-treated myometrium has not been fully elucidated with human lab studies. There are suggestions it may lead to increased oxytocin requirements or increased postpartum hemorrhage (PPH) in preeclamptic patients. This bears significance in the preeclamptic and eclamptic populations who are likely to be receiving MgSO4 in combination with oxytocin.

The investigators hypothesize that MgSO4 will reduce uterine contractions in oxytocin pre-treated myometrium, as well as untreated myometrium, and higher oxytocin doses will be needed to produce equivalent contractions. This will help us to better understand the implications of the use of these drugs together in the clinical setting.

MgSO4 is known to cause relaxation of uterine muscle in the doses used for preeclampsia and eclampsia and fetal neuroprotection. In these clinical settings, after delivery, oxytocin is often given to prevent or treat PPH. There is evidence that patients with preeclampsia already have a higher risk of PPH than non-preeclamptic patients. In the presence of the effects of MgSO4 treatment on the uterus, PPH could be worse in theory. If patients have had oxytocin-augmented labor, then the desensitization effect on the uterus also comes into play. Performing this study in vitro will allow us to assess the effects on the uterine muscle of MgSO4 in oxytocin-desensitized tissue samples.

Scientific evidence on pregnancy outcomes in these patients is timely and important. A study of myometrial contractility under controlled conditions is likely to provide us with information on the interactions between oxytocin and MgSO4 that occurs in many clinical scenarios, eliminating any confounders that could be encountered in clinical settings. The investigators hope to determine the myometrial contraction patterns induced by oxytocin in the presence of MgSO4, including in myometrial specimens pre-treated with oxytocin plus MgSO4, to mimic the clinical scenarios in preeclampsia and fetal neuroprotection. This will expand the depth of our understanding of the desensitization phenomenon, and could provide valuable insight into the mechanism responsible for PPH in these patient populations. This study will form a basis for future modifications in oxytocin augmentation regimens and pharmacotherapy for control of uterine atony and PPH in these patient populations being simultaneously treated with MgSO4.

Inclusion Criteria:

• Patients who give written consent to participate in this study
• Patients with gestational age 37-41 weeks
• Non-laboring patients, not exposed to exogenous oxytocin
• Patients requiring primary Cesarean delivery or first repeat Cesarean delivery under spinal anesthesia

Exclusion Criteria:

• Patients who refuse to give written informed consent
• Patients who require general anesthesia
• Patients who had previous uterine surgery or more than one previous Cesarean delivery
• Patients with any condition predisposing to uterine atony and postpartum hemorrhage, such as abnormal placentation, multiple gestation, preeclampsia, macrosomia, polyhydramnios, uterine fibroids, bleeding diathesis, chorioamnionitis, or a previous history of postpartum bleeding
• Emergency Cesarean section in labor
• Patients with medical/pregnancy related conditions, such as diabetes, preeclampsia and essential hypertension