Inducing and Augmenting Labour: Oxytocin and Prostaglandins

Two jobs: ripen the cervix, contract the uterus

Getting a baby out is two mechanical problems, and each needs a different drug. The uterus is two tissues doing opposite work. The body (fundus) is thick smooth muscle — the myometrium — whose job is to contract and push. The cervix at the bottom is a firm collar of connective tissue whose job, for nine months, is to stay shut and hold the pregnancy in. To deliver, the collar must first soften, thin and open (this is "ripening"), and then the muscle above must contract in a coordinated rhythm to drive the baby through it. Induction that ignores the order fails: pour on contractions against a hard, closed cervix and you get a painful, unproductive, and dangerous labour. So the pharmacology tracks the two jobs — prostaglandins to ripen the cervix, oxytocin to power the contractions — and the art is doing them in sequence.

Prostaglandins: ripening the cervix

Prostaglandins are the natural chemical signals of cervical remodelling — the same lipid mediators you meet in the Inflammation section, here doing reproductive rather than inflammatory work. Given as a vaginal preparation they soften the connective tissue of the cervix and, at the same time, stimulate myometrial contractions, so they can both ripen an unfavourable cervix and get labour going. Two agents dominate. Dinoprostone is PGE2, supplied as a vaginal gel, tablet or slow-release pessary — the standard, licensed cervical-ripening agent, with the advantage that a controlled-release pessary can simply be pulled out if the uterus is over-stimulated. Misoprostol is a PGE1 analogue: cheap, stable at room temperature (no refrigeration), and highly effective, which has made it the workhorse of induction worldwide, especially where cost and cold storage matter. Both share the whole-class hazard — push the dose and you get too many contractions.

Misoprostol is worth a moment's distinction, because you will meet it far beyond the term labour ward. The same drug that induces labour is used — in different doses and regimens — to manage early miscarriage (helping the uterus expel a failed or incomplete pregnancy) and, combined with the anti-progesterone mifepristone, for medical abortion: mifepristone first blocks progesterone to prime and soften the uterus, then misoprostol contracts it. It is the same fundamental pharmacology (a prostaglandin making the uterus contract) applied at different gestations and for different goals. The clinical context, not the molecule, changes.

Oxytocin: the mainstay for augmentation

Once the cervix is favourable, oxytocin is the drug that drives the labour. Oxytocin is a nine-amino-acid peptide made in the hypothalamus and released from the posterior pituitary — physiology you meet in the Endocrine section — and the synthetic version (Syntocinon) is the mainstay for two jobs: augmenting a labour that has started but slowed, and inducing labour once the cervix is favourable and the membranes have ruptured. Its mechanism is direct: it binds oxytocin receptors on the myometrium, triggering a rise in intracellular calcium that produces rhythmic, coordinated contractions. A crucial detail explains why it is a term drug and not an early-pregnancy one: the density of uterine oxytocin receptors climbs steeply towards the end of pregnancy, so the same dose that does little at 20 weeks produces powerful contractions at term. Because its effect is potent and the therapeutic window narrow, oxytocin is never given as a bolus for induction — it is a carefully titrated intravenous infusion, started low and increased in small steps while contractions and the fetal heart are watched, then dialled back the moment the uterus is doing enough.

The key safety issue: hyperstimulation and fetal distress

The commonest danger is not too little contraction but too much. The baby is oxygenated through the placenta, and the placenta is perfused between contractions — during a contraction, blood flow through the uterine wall is briefly squeezed off. A normal labour spaces contractions so the placenta refills each time. Push any uterotonic too hard and you get uterine hyperstimulation (tachysystole): contractions too frequent, too prolonged, or with too little relaxation between them. The uterus never gets the chance to refill the placenta, and the baby is progressively starved of oxygen — seen on the trace as fetal heart-rate decelerations and, if unchecked, distress. The response is immediate and mechanical: stop or reduce the uterotonic, reposition the mother and give oxygen and fluids, and if the uterus will not relax, give a tocolytic — typically terbutaline, a beta-2 agonist — to actively quiet the muscle. This is the direct link to the Tocolytics chapter: those drugs are the pharmacological opposite of everything here, used to relax a uterus rather than contract it, and terbutaline on the labour ward is kept precisely as the antidote to hyperstimulation.

Oxytocin carries a second, sneakier hazard that comes straight from its chemistry. Its structure is very close to that of vasopressin (antidiuretic hormone, ADH) — the two peptides differ by only a couple of amino acids — so at high doses oxytocin cross-activates vasopressin receptors in the kidney and exerts an antidiuretic effect: the kidney holds on to water. Give a high-dose, prolonged infusion made up in large volumes of hypotonic (dextrose) fluid, and the retained free water dilutes the blood, producing water intoxication and hyponatraemia — which in the extreme causes confusion, seizures and, in mother or newborn, real harm. The Endocrine section covers this vasopressin overlap in full; the practical lesson here is to run oxytocin in a concentrated form in isotonic saline, keep total fluid volumes modest, and watch for it in any long induction. A milder, more immediate effect is transient hypotension (with a reflex tachycardia) if oxytocin is pushed too fast — another reason it is infused slowly rather than injected.

When to induce — and when not to

Whether the cervix is ready is scored objectively by the Bishop score, which grades favourability from the cervix's dilatation, effacement (thinning), consistency, position, and the station of the baby's head. A high (favourable) score predicts that induction will succeed easily — often oxytocin and rupturing the membranes are enough; a low (unfavourable) score means the cervix must be ripened with a prostaglandin first. Alongside the drugs are mechanical methods that work by physical pressure rather than pharmacology: a balloon (Foley) catheter passed through the cervix and inflated to dilate it, and a membrane sweep, where an examining finger separates the membranes from the lower uterus to release local prostaglandins and encourage spontaneous labour. These are useful where a drug is contraindicated or best avoided.

The contraindications flow from one question: can this uterus safely contract hard? Because a uterotonic makes the uterus contract forcefully, it is dangerous wherever a forceful contraction would do harm. A uterus with a previous classical (vertical) caesarean scar or other major uterine surgery may rupture along the scar under strong contractions — a catastrophe for mother and baby — so induction is generally contraindicated. Malpresentation such as a transverse lie cannot deliver vaginally, so driving contractions is futile and dangerous. And a baby already showing signs of compromise (a non-reassuring fetal heart trace) should be delivered promptly, usually by caesarean, not subjected to the added stress of induced contractions. In each case the logic is the same: a uterotonic is only safe when a normal vaginal delivery is both possible and safe to pursue.