Spinal anaesthesia for caesarean section is the most commonly used anaesthetic choice in caesarean deliveries. This is usually associated with maternal hypotension and other adverse side effects. Prophylactic intravenous administration of ondansetron immediately and 5 minutes preoperatively have shown to provide a protective effect against hypotension while other studies have shown little effect on the incidence of blood pressure drop in healthy parturients.

The investigators will study the effect of different doses and timing of intravenous ondansetron in full term obstetric patients undergoing elective lower segment caesarean section under spinal anaesthesia on the incidence and severity of hypotension and other adverse side effects in healthy parturients having the standard intrathecal plain bupivacaine and fentanyl.

This prospective randomized double-blind placebo-controlled study will include 150 American Society of Anaesthesiologist physical status grade I (ASA I) pregnant women scheduled for elective caesarean delivery under spinal anaesthesia.

Patients will be randomly allocated into three groups (n = 50) to receive intravenous ondansetron 4mg in 10 mL normal saline (group O4), 6 mg in 10 mL normal saline(group O6), or placebo of 10 mL normal saline (group C), all 15-20 minutes before spinal anaesthesia.

Hemodynamic variables and other adverse effects will be assessed at 16 time points intraoperatively. Those variables include blood pressure, heart rate, oxygen saturation, nausea, vomiting, electrocardiographic changes, skin flushing, discomfort or pruritus, perioperative vasopressor requirements and patients' satisfaction.

Participants will be randomly allocated preoperatively as per randomizer.org software and will be blinded from other anaesthetists performing the spinal block and following patients intraoperatively.

Inclusion Criteria:

• patients who received spinal anesthesia for elective cesarean sections and meet the American Society of Anesthesiologist's (ASA) patient status classification I or II.

Exclusion Criteria:

• Patients ASA patient status classification III, IV or V
• Emergent cesarean sections
• Multiple parities (twins/triplets)
• More than 1,000 ml blood loss
• More than 6 mg ondansetron administered
• Patients presenting with a cardiac history (coronary artery disease, myocardial infarction, congestive heart failure, murmur, mitral valve prolapse/regurgitation, dysrhythmias, aortic stenosis/regurgitation)
• Patients presenting with preeclampsia
• Patients presenting for cesarean section with epidural due to failure to progress.

full-term pregnant females undergoing cesarean section.

No identifying information will be collected. Data will be used solely for statistical analysis.