Examqo › Examqo library › Obstetrics and gynaecology › Obstetrics · IMLE
Preterm labour
No drug holds back preterm labour for long; the goal is to buy 48 hours to give steroids for fetal lung maturity, magnesium for brain protection, and to transfer the mother to a centre with a neonatal unit.
Why it matters in the exam
Preterm birth is birth before 37 weeks and is the leading cause of neonatal morbidity and mortality. Exam questions describe a woman with regular contractions and cervical change and ask what to do now: betamethasone between 24 and 34 weeks, magnesium sulfate before 32 weeks, short tocolysis to allow time for steroids, and prophylaxis against group B streptococcus. You must also know when delay is forbidden: intra-amniotic infection, placental abruption, preeclampsia with severe features, fetal death or an abnormal fetal status.
The second half of the topic is prevention before contractions begin. Cervical length on transvaginal ultrasound between 16 and 24 weeks is the most important predictor. A woman with a short cervix receives vaginal progesterone, and a woman with a previous preterm birth and a short cervix is considered for a cerclage. A history of spontaneous preterm birth is the strongest risk factor, alongside multiple pregnancy, smoking, infection and a short interval between pregnancies. A negative fetal fibronectin test is useful mainly to rule out imminent delivery and allow discharge.
Key points
- Preterm birth = before 37 weeks; the strongest risk factor is a previous preterm birth.
- Betamethasone at 24 to 34 weeks to mature the lungs.
- Magnesium sulfate before 32 weeks to protect the brain.
- Tocolysis only for 48 hours and only without infection, abruption or fetal compromise.
- Short cervix before 24 weeks → vaginal progesterone, and in selected cases a cerclage.
Summary table
| Intervention | When | Why | Exam notes |
|---|---|---|---|
| Steroids (betamethasone) | 24 to 33+6 weeks, sometimes from 22 weeks | Mature the lungs and reduce RDS, intraventricular haemorrhage and NEC | 12 mg IM, 2 doses 24 hours apart; may be considered at 34–36+6 weeks if not given before |
| Magnesium sulfate | Before 32 weeks when delivery is expected | Fetal neuroprotection against cerebral palsy | Has no value as long-term tocolysis |
| Tocolysis | 24 to 34 weeks, for 48 hours | Time for steroids and transfer to a tertiary centre | Nifedipine or atosiban; indomethacin only before 32 weeks |
| Vaginal progesterone | Cervix 20 mm or less before 24 weeks | Reduces preterm birth in a singleton pregnancy | Not useful in twins |
| Cerclage | Previous spontaneous preterm birth and cervix below 25 mm before 24 weeks | Mechanical support of the cervix | Also history-indicated for cervical insufficiency |
| GBS prophylaxis | Preterm labour with unknown carrier status | Prevents neonatal sepsis | IV penicillin until delivery or until contractions stop |
In the full version for subscribers
- A simple algorithm
- Memory aid
- Exam traps
- 5 review cards
- Enough for IMLE
The full version is open to subscribers of: Medical licensing exam (IMLE), Obstetrics and gynecology, Pediatrics, Family medicine, ENT and head and neck surgery.
Subscriber? Read the full topic Plans and prices 25 free questions
Sources
- ACOG Practice Bulletin No. 171, 2016, reaffirmed — Management of Preterm Labor
- ACOG Practice Bulletin No. 234, 2021 — Prediction and Prevention of Spontaneous Preterm Birth
- ACOG Committee Opinion No. 713, 2017 — Antenatal Corticosteroid Therapy for Fetal Maturation
- Williams Obstetrics, 26th edition (2022) — Preterm Birth
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-02. The text may contain errors; when in doubt, check the source.