Postpartum hemorrhage (PPH) due to uterine atony is a major cause of maternal morbidity and mortality. Uterotonic drugs are used to improve the muscle tone of the uterus after birth and these are effective at reducing the incidence of PPH. Large doses of this drug are associated with adverse effects like lower blood pressure, nausea, vomiting, abnormal heart rhythms and changes on ECG. Various international bodies recommend varying and high doses of oxytocin in elective cesarean sections. A study performed at Mount Sinai Hospital showed that a much smaller doses of oxytocin is required (ED95 being 0.35IU). Women who had twins were excluded from this study. It is known that women with a twin pregnancy have a higher risk of poor tone and postpartum hemorrhage.

The investigators seek to find the best dose of oxytocin for the patients with a twin pregnancy. A higher dose may be needed to contract the uterus adequately.

Postpartum hemorrhage (PPH) is one of the leading causes of death during childbirth and accounts for an estimated 140,000 deaths per year worldwide. Furthermore, recent evidence has shown that the rate of PPH secondary to uterine atony is increasing.

Multiple pregnancy is a well-recognized risk factor for PPH. Compared with singleton pregnancy, women with a multiple pregnancy have an increased risk of PPH, severe PPH, transfusion, uterine atony, hysterectomy, prolonged hospital stay and death. This is true in both high- and low-income countries. Uterine atony as a cause of PPH is more likely in multiple pregnancy compared with singleton pregnancy.

Prophylactic uterotonic drugs administered after the delivery have been demonstrated to reduce the incidence of PPH by up to 40%. Oxytocin is the most commonly administered uterotonic drug used to prevent PPH in North America but is associated with adverse effects such as hypotension, nausea, vomiting, dysrhythmias, ST segment abnormalities, and severe water intoxication that may lead to pulmonary edema and convulsions.

Previous dose finding studies have excluded women with twin pregnancies. Therefore, the investigators wish to perform a double blinded dose finding study using the biased coin flip up-and-down sequential allocation technique to determine the ED 90 of oxytocin at cesarean section in those women with a twin pregnancy.

Inclusion Criteria:

• Twin pregnancy
• Elective cesarean delivery under regional anesthesia
• Gestational age ≥36 weeks
• No known additional risk factors for postpartum hemorrhage
• Written informed consent to participate in this study

Exclusion Criteria:

• Refusal to give written informed consent
• Allergy or hypersensitivity to oxytocin
• Conditions that may predispose to uterine atony and postpartum hemorrhage such as placenta previa, severe preeclampsia (as defined by SOGC guidelines (25)), polyhydramnios, uterine fibroids, previous history of uterine atony resulting in PPH, or bleeding diathesis and obesity, defined as pre-pregnancy BMI >40
• Hepatic, renal, and vascular disease
• Use of general anesthesia prior to the administration of the study drug