Preterm Labour Risk Assessment

Preterm labour is defined as regular uterine contractions occurring before 37 completed weeks of gestation, with or without cervical change. Preterm birth (PTB) — delivery before 37 weeks — is the leading cause of neonatal morbidity and mortality worldwide, responsible for approximately 70% of neonatal deaths and significant long-term neurodevelopmental disability including cerebral palsy, chronic lung disease, and cognitive impairment. Rates of preterm birth range from 5-7% in high-income countries to 15-18% in parts of sub-Saharan Africa and South Asia. Risk stratification tools identify women at highest risk who may benefit from preventive interventions. The most clinically validated predictors are: transvaginal ultrasound cervical length measurement — a cervical length below 25 mm at 16-24 weeks is associated with significantly elevated preterm birth risk, with women with a cervical length below 15 mm at 24 weeks having an approximately 80% rate of delivery before 32 weeks; and fetal fibronectin (fFN) assay — a protein found in the extracellular matrix at the choriodecidual interface, present in cervicovaginal secretions when the fetal membranes are beginning to detach. A positive fFN (>50 ng/mL or ≥200 ng/mL using quantitative assays) in women with symptoms of preterm labour predicts imminent preterm delivery with a positive likelihood ratio of approximately 5. The primary preventive intervention for identified high-risk women is vaginal progesterone (200 mg nightly) from 16-22 weeks, which reduces preterm birth risk by approximately 35% in women with a short cervix. Cervical cerclage is an option for women with cervical length below 25 mm in the context of a history of preterm birth or mid-trimester loss.

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1. 1Assess clinical risk factors at booking: prior preterm birth (strongest predictor — 15-30% recurrence risk), prior second trimester loss, cervical surgery (LLETZ, cone biopsy), multiple pregnancy, uterine anomaly (bicornuate, septate), smoking, substance misuse, short interpregnancy interval, and low socioeconomic status.
2. 2Offer transvaginal ultrasound (TVS) cervical length measurement at the anomaly scan (18-22 weeks) for all women with risk factors, or as part of universal screening programmes where adopted. Measure internal os to external os in the sagittal plane with the bladder empty, taking the shortest of three measurements.
3. 3Classify cervical length risk: normal ≥25 mm; intermediate 20-24 mm (monitoring and progesterone consideration); short <25 mm (progesterone); very short <15 mm (progesterone + consider cerclage if risk factors).
4. 4Perform fetal fibronectin (fFN) swab in women with symptoms of preterm labour (regular contractions, pelvic pressure) at 22-34 weeks with intact membranes and cervical dilation <3 cm. Collect from the posterior fornix before any digital examination. Result available in <30 minutes.
5. 5Interpret fFN: negative (<50 ng/mL) = high negative predictive value for delivery within 7 days (>99% negative predictive value); positive (≥50 ng/mL or ≥200 ng/mL quantitative) = positive prediction of preterm delivery within 7-14 days is moderate.
6. 6Initiate prevention for high-risk women: vaginal progesterone 200 mg (Cyclogest or equivalent) nightly from 16-22 weeks, continued to 34-36 weeks; cervical cerclage for women with cervical length <25 mm and a history of prior preterm birth or loss; and consider cervical pessary as an alternative to cerclage in some settings.
7. 7When preterm labour is established or imminent: administer antenatal corticosteroids (betamethasone 12 mg IM × 2 doses 24 hours apart, or dexamethasone equivalent) before 34+6 weeks for fetal lung maturity; magnesium sulfate before 32 weeks for fetal neuroprotection; and consider tocolysis for 48 hours to complete the corticosteroid course.

• !Using transabdominal instead of transvaginal ultrasound for cervical length measurement — TVS consistently gives shorter, more accurate measurements; transabdominal overestimates cervical length and misses short cervices.
• !Performing fFN swab after digital examination — digital exam disrupts the fFN assay, causing false positives; fFN must always be collected before any vaginal examination.
• !Admitting and treating all women with a positive fFN — positive fFN has moderate positive predictive value; management should be based on the combination of fFN, cervical length, clinical symptoms, and gestational age.
• !Prescribing cervical cerclage in multiple pregnancy without clear prior PTB history — cerclage in multiples without prior preterm birth increases rather than decreases PTB risk.
• !Forgetting magnesium sulfate for neuroprotection when delivering prematurely before 32 weeks — this is frequently omitted in emergency situations and is a preventable cause of cerebral palsy.
• !Not rescreening cervical length after an initially normal measurement in a high-risk woman — cervical shortening can occur rapidly and missed shortening leads to missed treatment opportunities.