Uterine stimulants (uterotonics, oxytocics) increase the frequency and strength of myometrial contractions or soften (ripen) the cervix.

• Oxytocin binds oxytocin receptors on myometrial cells, raising intracellular calcium. Receptor numbers rise sharply near term, so the uterus becomes very sensitive in late pregnancy. Oxytocin is structurally close to antidiuretic hormone and, at high doses, retains water
• Prostaglandins — misoprostol (PGE1) and dinoprostone (PGE2) break down cervical collagen (ripening) and stimulate contractions. Carboprost (15-methyl PGF2α) produces strong, sustained uterine contraction
• Ergot alkaloids — methylergonovine causes sustained (tetanic) uterine contraction and constricts blood vessels; therefore it is used only after birth, never for labor

Tocolytics relax the uterus to delay preterm birth long enough (about 48 hours) for antenatal corticosteroids and maternal transfer.

• Nifedipine (calcium channel blocker) blocks calcium entry into smooth muscle
• Indomethacin (NSAID) blocks prostaglandin synthesis
• Terbutaline (beta-2 agonist) raises cAMP and relaxes smooth muscle; also used for acute tachysystole
• Magnesium sulfate competes with calcium; its main obstetric uses today are seizure prophylaxis in preeclampsia and fetal neuroprotection before 32 weeks. It is not a first-choice tocolytic

No tocolytic prevents preterm birth for long or improves newborn outcomes on its own; the benefit comes from the time gained.

Uterine stimulants

Tranexamic acid is not a uterotonic but is given with them in PPH: 1 g IV over 10 minutes within 3 hours of birth, with a second 1 g dose if bleeding continues after 30 minutes or restarts within 24 hours. Dinoprostone and carboprost labels restrict use to hospitals with trained staff and surgical capability.

Tocolytics and related drugs in preterm labor

Atosiban (an oxytocin receptor antagonist) is used as a tocolytic in some countries but is not FDA-approved.

Oxytocin

• Tachysystole (more than 5 contractions in 10 minutes, averaged over 30 minutes) → reduced placental perfusion, FHR decelerations, uterine rupture
• Water intoxication (hyponatremia) with high doses and large volumes of hypotonic fluid: headache, nausea, confusion, seizures
• Hypotension and tachycardia with rapid undiluted IV injection
• Postpartum hemorrhage from uterine fatigue after prolonged infusion

Prostaglandins: tachysystole, nausea, vomiting, diarrhea, fever and shivering (especially misoprostol), bronchospasm (carboprost), flushing.

Methylergonovine: severe hypertension, headache, stroke, seizures, coronary spasm, nausea.

Nifedipine: maternal hypotension, headache, flushing, dizziness, reflex tachycardia.

Indomethacin: maternal GI upset and bleeding; fetal premature ductus closure, oligohydramnios (reduced fetal urine), and, in the newborn, possible necrotizing enterocolitis and intraventricular hemorrhage with prolonged use.

Terbutaline: maternal tachycardia, palpitations, tremor, hyperglycemia, hypokalemia, chest pain, dysrhythmias, pulmonary edema; fetal tachycardia; neonatal hypoglycemia.

Magnesium sulfate: expected — flushing, warmth, nausea, drowsiness, muscle weakness. Toxic — loss of reflexes, respiratory depression, cardiac arrest. Newborn may be drowsy and hypotonic. Continuous use longer than 5–7 days can cause fetal hypocalcemia and bone abnormalities (FDA). Magnesium relaxes the uterus → postpartum atony.

Betamethasone: transient maternal hyperglycemia (monitor in diabetes) and a temporary fall in fetal heart rate variability.

Oxytocin and induction in general: contraindicated when vaginal birth is contraindicated — placenta previa or vasa previa, transverse lie, prior classical uterine incision, active genital herpes, cord prolapse; also category III FHR tracing needing immediate birth. Hypertonic uterine dysfunction is not treated with oxytocin.

Misoprostol: not for cervical ripening or induction with a previous cesarean or major uterine surgery (rupture); postpartum use for PPH is not restricted by a scar. Contraindicated in pregnancy when used for ulcer prevention (causes abortion).

Dinoprostone: caution with asthma, glaucoma, previous cesarean; do not start oxytocin until about 30 minutes after removing the insert (6–12 hours after gel); oxytocin may start about 4 hours after the last misoprostol dose.

Methylergonovine: hypertension, preeclampsia, heart disease, peripheral vascular disease; strong CYP3A4 inhibitors (HIV protease inhibitors, clarithromycin, azole antifungals) → ergotism.

Carboprost: asthma; caution with heart, liver, or kidney disease.

Tocolytics in general are contraindicated with intrauterine fetal death, lethal anomaly, nonreassuring fetal status, severe preeclampsia or eclampsia, maternal hemorrhage with instability, chorioamnionitis, and preterm labor beyond 34 weeks.

Nifedipine: maternal hypotension, cardiac disease with preload dependence; use together with magnesium only with close monitoring (additive hypotension and neuromuscular effects).

Indomethacin: 32 weeks or later, oligohydramnios, maternal bleeding disorder, peptic ulcer, kidney or liver disease, NSAID-sensitive asthma.

Terbutaline: maternal heart disease, tachydysrhythmias, hyperthyroidism, poorly controlled diabetes.

Magnesium sulfate: myasthenia gravis; reduce dose with kidney impairment (renally excreted); potentiates neuromuscular blockers.

1. Oxytocin safety (high-alert)
   • Dilute in isotonic fluid; give by infusion pump as a secondary line piggybacked at the port closest to the client, so it can be stopped at once while IV access continues
   • Low-dose regimens commonly start at 0.5–2 mU/min and increase by 1–2 mU/min every 15–40 minutes; maximum per facility protocol
   • Continuous electronic FHR and contraction monitoring; assess frequency, duration, intensity, and resting tone
   • Tachysystole with abnormal FHR: stop or reduce oxytocin, turn to the side, give an IV fluid bolus, notify the provider, give terbutaline if ordered
   • Record intake and output; watch for water intoxication
2. Magnesium sulfate safety (at least hourly)
   • Deep tendon reflexes, respiratory rate (at least 12/min), urine output (at least 30 mL/h), level of consciousness, SpO₂, lung sounds
   • Serum magnesium when toxicity is suspected or creatinine rises; therapeutic about 4–8 mEq/L (4.8–9.6 mg/dL; 2–4 mmol/L)
   • Pump on a secondary line; calcium gluconate and resuscitation equipment at the bedside
   • After birth: check fundus and bleeding (atony risk)
3. Before postpartum uterotonics: check blood pressure before methylergonovine (hold and report hypertension); ask about asthma before carboprost; massage the fundus; record time of birth for the TXA window
4. Nifedipine: blood pressure and pulse before each dose; hold for hypotension per order
5. Terbutaline: maternal heart rate (commonly held if above about 120/min per protocol), chest pain, lung sounds, glucose, potassium
6. Indomethacin: confirm gestational age under 32 weeks and limit to 48 hours; give with food; assess amniotic fluid as ordered
7. Betamethasone: deep IM; monitor glucose in diabetes
8. Prostaglandin ripening: continuous monitoring after placement; remove the dinoprostone insert for tachysystole, abnormal FHR, or active labor

• Oxytocin may make contractions stronger quickly — report severe continuous pain or pressure; pain relief options remain available
• Magnesium commonly causes warmth, flushing, and tiredness — report difficulty breathing, chest pain, or blurred vision
• Terbutaline may cause a racing heart or shakiness — report chest pain or shortness of breath
• Preterm labor warning signs (before 37 weeks): regular contractions or tightening (commonly 4 or more in an hour), low dull backache, pelvic pressure, menstrual-like cramps, change in vaginal discharge, leaking fluid — go to the hospital
• After methylergonovine: report severe headache or chest pain
• After misoprostol: fever and shivering are common and short-lived

Loss of reflexes appears before respiratory depression — checking reflexes catches magnesium toxicity early.

• Oxytocin = high-alert: pump, secondary line at the proximal port, continuous FHR monitoring
• Tachysystole = more than 5 contractions in 10 minutes → stop or reduce oxytocin, lateral, fluids, notify
• Oxytocin can cause water intoxication; never an undiluted rapid IV push
• Misoprostol: no cervical ripening or induction with a prior cesarean scar
• Methylergonovine: check BP — no hypertension/preeclampsia; carboprost: no asthma
• Tocolysis buys 48 hours for betamethasone (24–34 weeks) and transfer
• Indomethacin: under 32 weeks, 48 hours max (ductus, oligohydramnios)
• Terbutaline boxed warning: no prolonged injectable tocolysis; no oral terbutaline for preterm labor
• Magnesium before 32 weeks = fetal neuroprotection
• Magnesium monitoring: reflexes, RR ≥ 12, urine ≥ 30 mL/h; toxicity → stop and give calcium gluconate 1 g IV