Cervical Funneling

Cervical funnelling is dilatation of the internal cervical os with the membranes bulging into the upper cervical canal, seen on ultrasound as a T, Y, V or U shaped canal, with the funnel making up 25 percent or more of the total cervical length. It results from cervical weakness or from contractions pushing on the cervix. It matters because it often accompanies a short cervix and carries a high risk of preterm birth when the remaining closed length is short.

• A funnel with a closed length of 25 mm or more carries a low risk.
• Risk factors: previous mid-trimester loss, cervical surgery, previous preterm birth, twin pregnancy, uterine anomaly.
• A U shape with advanced dilatation is the highest risk.

Many women have no symptoms. Others have pelvic pressure, increased discharge, backache or painless mid-trimester dilatation with prolapsing membranes.

• Found on routine ultrasound.
• Membranes may be visible in the vagina at speculum examination.
• Differential diagnosis: lower segment contraction, short cervix, cervical insufficiency, early labour, hourglass membranes.
• Complications: preterm labour, mid-trimester loss, preterm rupture of membranes, chorioamnionitis, neonatal death.

The approach is transvaginal ultrasound with measurement of the funnel and the closed length.

• Transvaginal ultrasound: measure the funnel width (5 mm or more is significant), funnel length, and the remaining closed canal; repeat over 5 minutes and after gentle fundal pressure, because contractions can create a false funnel.
• Check for sludge, bulging membranes and liquor volume.
• Urine culture, high vaginal swab, chlamydia and gonorrhoea tests, syphilis test; full blood count and C-reactive protein.
• Amniocentesis for glucose, white cells and culture in specialist units when rescue cerclage is considered.

The principle is to treat by the length of closed canal, using progesterone, cerclage and rescue cerclage when the cervix is dilating and infection is excluded.

• Supportive: stop contractions by hydration and rest; avoid heavy lifting and intercourse; no digital examination; teach warning signs; admit for dilatation or bulging membranes; left lateral or head down position for a prolapsing sac.
• Pharmacological: micronised progesterone 200 mg vaginal at night to 36 weeks for a closed length below 25 mm; indomethacin 100 mg rectal once then 25 mg oral 6-hourly for 48 hours (before 32 weeks only) and cefazolin 2 g IV once around rescue cerclage; metronidazole 400 mg oral 12-hourly for 7 days for bacterial vaginosis; betamethasone 12 mg IM twice, 24 hours apart, between 24 weeks and 34 weeks and 6 days if labour starts.
• Procedures: McDonald cerclage at 16 to 24 weeks for a closed length below 25 mm with a previous preterm birth or mid-trimester loss; rescue cerclage at 16 to 26 weeks for a dilated cervix with bulging membranes in the absence of contractions, bleeding or infection; remove at 36 to 37 weeks or at once for labour, rupture of membranes or infection.
• Red flags: bulging membranes at the vulva, contractions, fever, foul discharge, leaking fluid, bleeding.
• Follow-up: cervical length and funnel every 1 to 2 weeks to 28 weeks; antenatal review every 2 weeks; plan birth in a unit with neonatal care.