Oxytocin is secreted by the posterior pituitary gland. In pregnancy, it acts on the uterus resulting in uterine contraction. It also causes contraction of the milk ducts. In clinical practice, synthetic oxytocin can be given as an infusion during labor to augment uterine contractions, hence facilitating delivery. However hyperstimulation of the uterus can result in fetal compromise as the blood flow to the placenta will be affected by a persistently contracted uterus. Special precaution should be taken when oxytocin is used in parturients with cardiac disease as severe hypotension can occur. This is due to the vasodilatory effect of oxytocin.

In addition to stimulating/augmenting uterine contractions, oxytocin also has an anti-diuretic effect because of its structural similarity to vasopressin. Prolonged usage or high doses of oxytocin can cause potentially fatal water intoxication and hyponatremia. This is further exacerbated if it is infused in dextrose solutions.

When used for the induction of labor, an intravenous infusion is commenced and the dosage is increased at intervals of 30 min. The aim is to achieve 3–4 contractions every 10 min. The maximum rate is 0.02 units/min with not more than 5 units being administered over a 24-h period.

At delivery, oxytocin is used to prevent postpartum hemorrhage. A slow intravenous dose is given when the anterior shoulder is delivered or an intramuscular injection given after delivery of the fetus. An intravenous infusion after delivery can also be administered to treat postpartum hemorrhage.

Carbetocin is a long-acting synthetic analogue of oxytocin. It can be administered intravenously as a single dose immediately following delivery, to prevent uterine atony and postpartum hemorrhage. Although less potent than oxytocin, carbetocin has a longer duration of action lasting up to about 1 h. Because it produces prolonged uterine contraction, carbetocin must not be administered before delivery of the infant. Its side effects are similar to those of oxytocin.

Ergometrine is another drug which stimulates uterine contraction. A combination of oxytocin and ergometrine, known as Syntometrine, is given intramuscularly after delivery for the prevention of postpartum hemorrhage. Its side effects of severe nausea and vomiting make it a less favored drug. Furthermore, a marked rise in blood pressure can be observed due to vasoconstriction which can last for several hours. Hence, it can be hazardous if used in parturients with preeclampsia or cardiovascular diseases. Ergometrine 250–500 micrograms can be given by the intramuscular or intravenous route for the treatment of postpartum hemorrhage.

Uterine relaxation is due to the β-adrenoceptor agonist effect. The drug binds to β-adrenoceptors which results in activation of the enzyme adenylate cyclase. This increases the levels of cAMP and causes a reduction of intracellular calcium level, resulting in uterine relaxation. Treatment for premature labor is via an intravenous infusion which is given in incremental doses at intervals of 10 min. The end point is when contractions diminish. The infusion rate is then further increased (maximum infusion of 45 μg/min) until contractions have ceased. This infusion rate is maintained for 1 h after which the infusion is gradually reduced. Side effects include tremor, hypotension, wheezing, tachycardia (at high doses), and hypoglycemia.

Nifedepine is given orally. The drug blocks the influx of calcium ions, resulting in uterine relaxation. Its side effects are headache, dizziness, constipation, and wheezing.

These drugs act by depleting the amount of prostaglandins that initiate uterine stimulation. Examples of these drugs are indomethacin, aspirin and ibuprofen. Caution must be used with these drugs as they can result in premature closure of the ductus arteriosus in the fetus.