Cervical Shortening

Cervical shortening is a progressive fall in cervical length on serial transvaginal scans, for example a fall of more than 5 mm over 2 weeks or from 30 mm to below 25 mm, in the mid-trimester. It reflects early cervical ripening from infection, inflammation, insufficiency or uterine overdistension. It matters because the rate of change adds to the risk shown by a single measurement, and early action can prevent preterm birth.

• Normal cervical length at 18 to 24 weeks is about 35 to 40 mm and stays stable until about 32 weeks.
• Risk factors: previous preterm birth, mid-trimester loss, cervical surgery, twin pregnancy, bleeding, genital infection.
• Surveillance is needed in women with risk factors.

Most women have no symptoms. Some have pelvic pressure, mucoid discharge, backache or occasional contractions.

• Found only on serial ultrasound.
• Digital examination often normal.
• Differential diagnosis: measurement variation, lower segment contraction, normal late pregnancy change, cervical funnelling, early labour.
• Complications: short cervix, preterm labour, preterm rupture of membranes, mid-trimester loss, neonatal death.

The approach is serial transvaginal ultrasound using one technique each time.

• Transvaginal ultrasound every 1 to 2 weeks in women with risk factors from 16 to 24 weeks, with an empty bladder and three measurements of the closed canal.
• A fall of more than 5 mm in 2 weeks, or a length that crosses 25 mm, is significant.
• Urine culture, high vaginal swab, chlamydia and gonorrhoea tests, syphilis test.
• Check for funnelling, sludge and membrane bulging.

The principle is to increase surveillance, treat infection and start progesterone when the cervix is below 25 mm, adding cerclage when risk factors are present.

• Supportive: scans every 1 to 2 weeks; avoid heavy lifting; treat infection; stop smoking; teach warning signs; admit for contractions or bulging membranes.
• Pharmacological: micronised progesterone 200 mg vaginal at night from the first length below 25 mm 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 if labour starts.
• Procedures: McDonald cervical cerclage at 16 to 24 weeks when the length falls below 25 mm and there is a previous spontaneous preterm birth or mid-trimester loss, with cefazolin 2 g IV once and indomethacin 100 mg rectal once; remove at 36 to 37 weeks or at once for labour, rupture of membranes, infection or bleeding; Arabin pessary where available.
• Red flags: length below 15 mm, funnelling, contractions, leaking fluid, bleeding, fever.
• Follow-up: cervical length every 1 to 2 weeks until 24 weeks, then every 2 to 4 weeks to 28 weeks; antenatal review every 2 weeks.