Short Cervix in Pregnancy

A short cervix in pregnancy is a cervical length of less than 25 mm measured by transvaginal ultrasound between 16 and 24 weeks, with or without symptoms. It results from cervical insufficiency, previous cervical surgery, uterine anomaly or early inflammation. It is the strongest single predictor of spontaneous preterm birth, and treatment lowers the risk in the right women.

• The risk of birth before 34 weeks rises as the cervix gets shorter.
• Risk factors: previous preterm birth or mid-trimester loss, cone biopsy, loop excision, cervical trauma, uterine anomaly, multiple pregnancy, previous caesarean section at full dilatation.
• Vaginal progesterone lowers preterm birth in a singleton pregnancy.

Most women have no symptoms. Some have pelvic pressure, increased watery or mucoid discharge or painless dilatation and bulging membranes.

• Often found at the anomaly scan or surveillance scan for risk factors.
• Digital examination may be normal.
• Differential diagnosis: normal variation, lower uterine segment contraction, cervical funnelling, early labour, uterine fibroid affecting measurement.
• Complications: preterm labour, preterm rupture of membranes, mid-trimester loss, chorioamnionitis, neonatal death.

The approach is transvaginal ultrasound with a correct technique.

• Transvaginal ultrasound with an empty bladder, measuring the closed endocervical canal in a straight line over 3 measurements and recording the shortest; repeat after 30 to 60 seconds of gentle fundal pressure to show dynamic change.
• Look for funnelling, sludge in the liquor and bulging membranes.
• Urine culture, high vaginal swab for bacterial vaginosis, chlamydia and gonorrhoea tests, syphilis test.
• Where transvaginal ultrasound is unavailable, use the history of painless mid-trimester loss or previous preterm birth to decide on cerclage and progesterone.

The principle is vaginal progesterone for all women with a short cervix and cerclage for those with a previous preterm birth or mid-trimester loss.

• Supportive: avoid heavy lifting and intercourse if the cervix is below 15 mm; treat infection; stop smoking; teach warning signs; admit if contractions or bulging membranes develop; no strict bed rest.
• Pharmacological: micronised progesterone 200 mg vaginal at night from diagnosis to 36 weeks; metronidazole 400 mg oral 12-hourly for 7 days for bacterial vaginosis; cefalexin 500 mg oral 6-hourly for 7 days for bacteriuria; azithromycin 1 g oral once for chlamydia; betamethasone 12 mg IM twice, 24 hours apart, between 24 weeks and 34 weeks and 6 days when preterm labour starts.
• Procedures: McDonald cervical cerclage at 16 to 24 weeks for a cervical length below 25 mm with a previous spontaneous preterm birth or mid-trimester loss, with cefazolin 2 g IV once and indomethacin 100 mg rectal once; remove the cerclage at 36 to 37 weeks or at once for labour, rupture of membranes, infection or bleeding; Arabin pessary where available as an alternative.
• Special groups: twin pregnancy with a short cervix does not benefit from cerclage and progesterone is of unproven value, so surveillance and steroids when labour starts are the main care.
• Red flags: contractions, bulging membranes, leaking fluid, bleeding, fever, cervical length below 15 mm.
• Follow-up: cervical length every 1 to 2 weeks to 24 weeks; antenatal review every 2 weeks; plan birth in a unit with neonatal care.