Tocolytics

Introduction & therapeutic rationale

• Tocolytics (uterine relaxants) are drugs that decrease myometrial motility, used to delay or postpone preterm labour and (historically) to arrest threatened abortion.
• Preterm birth = delivery before 37 completed weeks; complicates >10% of pregnancies and is the dominant context for tocolysis. The more premature the neonate, the greater the risk of RDS, pulmonary hypertension and intracranial haemorrhage.
• Risk factors for preterm labour — multifetal gestation, premature rupture of membranes (PROM), intrauterine infection, and placental insufficiency.
• The goal is to DELAY, not prevent prematurity — the bought time is used to (1) transfer the mother in utero to a tertiary centre with neonatal facilities, and (2) administer antenatal glucocorticoids for fetal lung maturation (± antibiotics for PROM with infection to reduce neonatal group-B streptococcal disease).
• Central efficacy caveat (the key exam point) — tocolytics delay delivery in ~80% of women (~70% for ritodrine) but neither prevent premature birth nor improve fetal outcome (e.g. RDS); the superiority of any one agent over another is unestablished.