• Need for uterine massage: questionnaire [ Time Frame: 20 minute ]
  The obstetricians will be asked if there was any need for uterine massage beyond the initial 3 minute evaluation period following delivery.
• Intraoperative requirement for additional uterotonic medication [ Time Frame: 1 hour ]
  A request made by the obstetrician performing the cesarean delivery for additional uterotonic medication, due to bleeding or poor uterine tone.
• Calculated estimate of blood loss [ Time Frame: 24 hours ]
  Blood loss will be calculated through the difference in hematocrit values assessed prior to and at the end of 48 hours after the cesarean delivery, according to the following formula: Calculated blood loss = EBV ((Pre-op Htc-Post-op Htc)/pre-op Htc). EBV (estimated blood volume) in ml: patient's weight in kg x 85
• Intravenous fluid administered during surgery [ Time Frame: 2 hours ]
  The total volume (ml) of fluid administered from entering the operating room to skin closure.
• Hypotension: systolic blood pressure less than 80% of baseline [ Time Frame: 2 hours ]
  Systolic blood pressure < 80% of baseline, from drug administration until end of surgery
• Tachycardia: heart rate greater than 130% of baseline [ Time Frame: 2 hours ]
  Heart rate > 130% of baseline, from drug administration until end of surgery
• Bradycardia: heart rate less than 70% of baseline [ Time Frame: 2 hours ]
  Heart rate < 70% of baseline or a heart rate < 50bpm, from drug administration until end of surgery
• Presence of ventricular tachycardia: ECG [ Time Frame: 2 hours ]
  Presence of ventricular tachycardia as recorded by ECG, from drug administration until end of surgery
• Presence of atrial fibrillation: ECG [ Time Frame: 2 hours ]
  Presence of atrial fibrillation as recorded by ECG, from drug administration until end of surgery
• Presence of atrial flutter: ECG [ Time Frame: 2 hours ]
  Presence of atrial flutter as recorded by ECG, from drug administration until end of surgery
• Presence of nausea: questionnaire [ Time Frame: 2 hours ]
  The presence of nausea and number of episodes, from drug administration until end of surgery, as reported by the patient
• Presence of vomiting: questionnaire [ Time Frame: 2 hours ]
  The presence of vomiting and number of episodes, from drug administration until end of surgery
• Presence of chest pain: questionnaire [ Time Frame: 2 hours ]
  Any presence of chest pain, from drug administration until end of surgery, as reported by the patient
• Presence of shortness of breath: questionnaire [ Time Frame: 2 hours ]
  Any presence of shortness of breath, from drug administration until end of surgery, as reported by the patient
• Presence of headache: questionnaire [ Time Frame: 2 hours ]
  Any presence of headache, from drug administration until end of surgery, as reported by the patient
• Presence of flushing: questionnaire [ Time Frame: 2 hours ]
  Any presence of flushing, from drug administration until end of surgery

• Need for uterine massage [ Time Frame: 20 minute ]
  The need for uterine massage beyond the initial 3 minute evaluation period following delivery.
• Intraoperative requirement for additional uterotonic medication [ Time Frame: 1 hour ]
  A request made by the obstetrician performing the cesarean delivery for additional uterotonic medication, due to bleeding or poor uterine tone.
• Calculated estimate of blood loss [ Time Frame: 24 hours ]
  Blood loss will be calculated through the difference in hematocrit values assessed prior to and at the end of 48 hours after the cesarean delivery, according to the following formula: Calculated blood loss = EBV ((Pre-op Htc-Post-op Htc)/pre-op Htc). EBV (estimated blood volume) in ml: patient's weight in kg x 85
• Intravenous fluid administered during surgery [ Time Frame: 2 hours ]
  The total volume (ml) of fluid administered from entering the operating room to skin closure.
• Hypotension: systolic blood pressure less than 80% of baseline [ Time Frame: 2 hours ]
  Systolic blood pressure < 80% of baseline, from drug administration until end of surgery
• Tachycardia: heart rate greater than 130% of baseline [ Time Frame: 2 hours ]
  Heart rate > 130% of baseline, from drug administration until end of surgery
• Bradycardia: heart rate less than 70% of baseline [ Time Frame: 2 hours ]
  Heart rate < 70% of baseline or a heart rate < 50bpm, from drug administration until end of surgery
• Presence of ventricular tachycardia: ECG [ Time Frame: 2 hours ]
  Presence of ventricular tachycardia as recorded by ECG, from drug administration until end of surgery
• Presence of atrial fibrillation: ECG [ Time Frame: 2 hours ]
  Presence of atrial fibrillation as recorded by ECG, from drug administration until end of surgery
• Presence of atrial flutter: ECG [ Time Frame: 2 hours ]
  Presence of atrial flutter as recorded by ECG, from drug administration until end of surgery
• Presence of nausea: questionnaire [ Time Frame: 2 hours ]
  The presence of nausea and number of episodes, from drug administration until end of surgery, as reported by the patient
• Presence of vomiting: questionnaire [ Time Frame: 2 hours ]
  The presence of vomiting and number of episodes, from drug administration until end of surgery
• Presence of chest pain: questionnaire [ Time Frame: 2 hours ]
  Any presence of chest pain, from drug administration until end of surgery, as reported by the patient
• Presence of shortness of breath: questionnaire [ Time Frame: 2 hours ]
  Any presence of shortness of breath, from drug administration until end of surgery, as reported by the patient
• Presence of headache: questionnaire [ Time Frame: 2 hours ]
  Any presence of headache, from drug administration until end of surgery, as reported by the patient
• Presence of flushing: questionnaire [ Time Frame: 2 hours ]
  Any presence of flushing, from drug administration until end of surgery

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. Oxytocin is the most commonly used uterotonic drug to prevent and treat PPH. Large doses of this drug are asociated with adverse effects like low 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 dose of oxytocin is required (ED95 being 0.35IU). However, most of the women included in this study were below a body mass index (BMI) of 40kg/m2.

The investigators seek to find the best dose for patients with a BMI>40kg/m2, as a higher dose may be needed in this population 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. The World Health Organization (WHO) recommends active management of the third stage of labor to prevent PPH, even in low risk patients. Prophylactic uterotonic drugs administered after delivery are the main element of active management of the third stage and have been demonstrated to reduce the incidence of PPH by up to 40%. Oxytocin is the most commonly used uterotonic in North America, however it has a very short duration of action and requires a continuous infusion to achieve sustained effect, with large doses associated with adverse effects like low blood pressure, nausea, vomiting, abnormal heart rhythms and changes on ECG.

The prevalence of obesity is increasing in young women and some studies have shown that obese women have higher rates of caesarean delivery compared to non-obese women. Other studies have demonstrated an increased risk of hemorrhage due to poor uterine tone in obese women. Laboratory studies show that BMI alone appears to contribute to blunted uterine muscle responses and therefore contraction responses to oxytocin in obese women.

Previous dose finding studies have excluded those women with a BMI of ≥40kgm2. Therefore, the investigators wish to perform a double-blind dose finding study using the biased coin up-and-down sequential allocation technique to determine the ED90 of oxytocin at cesarean section in those women with a BMI>40.

• Postpartum Hemorrhage
• Obesity

• Active Comparator: Oxytocin 0.5IU
  Patient is given 0.5IU of oxytocin intravenously over 1 minute, immediately upon delivery of the fetal head.
  Intervention: Drug: Oxytocin
• Active Comparator: Oxytocin 1IU
  Patient is given 1IU of oxytocin intravenously over 1 minute, immediately upon delivery of the fetal head.
  Intervention: Drug: Oxytocin
• Active Comparator: Oxytocin 2IU
  Patient is given 2IU of oxytocin intravenously over 1 minute, immediately upon delivery of the fetal head.
  Intervention: Drug: Oxytocin
• Active Comparator: Oxytocin 3IU
  Patient is given 3IU of oxytocin intravenously over 1 minute, immediately upon delivery of the fetal head.
  Intervention: Drug: Oxytocin
• Active Comparator: Oxytocin 4IU
  Patient is given 4IU of oxytocin intravenously over 1 minute, immediately upon delivery of the fetal head.
  Intervention: Drug: Oxytocin
• Active Comparator: Oxytocin 5IU
  Patient is given 5IU of oxytocin intravenously over 1 minute, immediately upon delivery of the fetal head.
  Intervention: Drug: Oxytocin

Inclusion Criteria:

• BMI ≥40kg/m2
• Elective cesarean delivery under regional anesthesia
• Gestational age ≥ 37 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 (other than high BMI) that may predispose to uterine atony and postpartum hemorrhage such as placenta previa, multiple gestation, severe preeclampsia (as defined by SOGC guidelines (21)), eclampsia, polyhydramnios, uterine fibroids, previous history of uterine atony resulting in PPH, or bleeding diathesis.
• Hepatic, renal, and vascular disease
• Use of general anesthesia prior to the administration of the study drug