Cervical incompetence refers to a painless spontaneous dilatation of the cervix and is a common cause of second trimester pregnancy failure.
The estimated incidence varies geographically and generally thought to be around 1-1.5% of all pregnancies .
Typically cervical incompetence manifests in the second trimester. Patients at high risk for preterm delivery include those with:
- idiopathic (most common)
- uterine anomalies
- exposure to diethylstilbestrol (DES)
- previous cervical trauma or surgery
- previous recurrent spontaneous or therapeutic abortion
- previous premature delivery
- multifetal pregnancy
- connective tissue disorder (Ehlers-Danlos syndrome)
- fetal fibronectin (fFN) test on vaginal mucus: needs to be done before trans-vaginal scanning is attempted
Transvaginal, translabial or transperineal sonography is most commonly used to assess the cervix.
It should be emphasized that the appearance of the cervix may change during the examination and therefore multiple observations are recommended. Excessive pressure with the probe and an overly distended bladder may give false reassurance by artificially lengthening and narrowing the cervical canal. It is also useful to measure the worst finding.
Transvaginal scanning is required. In a late 1trimester scan, an opening of the cervical os at rest or in response to fundal pressure is considered an early feature . Findings include:
- bulging of the fetal membranes into a widened internal os (considered the most reliable sign )
- the appearance of this can worsen from a T-shape to a Y-shape to a V-shape and finally to a U-shape (see cervical incompetence mnemonic)
- if there is complete bulging, it can give an hourglass-type appearance
- shortening of the cervical canal
- in severe cases, there may be fetal parts or umbilical cord that extend through the os
It is used as a prognostic indicator for the risk of preterm labor progressing into preterm delivery.
The cervical length (CL) is obtained by measuring the endocervical canal from the internal cervical os to the external cervical os.
The normal cervix should be at least 30 mm in length. Cervical incompetence is variably defined, however, a cervical length of <25 mm at or before 24 weeks is often used. The risk of preterm delivery is inversely proportional to cervical length :
- 18% for <25 mm
- 25% for <20 mm
- 50% for <15 mm
In borderline cases, transfundal pressure may be used to confirm the diagnosis.
The presence of cervical funnelling is also an important finding. Greater than 50% funnelling before 25 weeks is associated with 80% risk of preterm delivery.
Sonographic determination of the residual closed length of the cervix may be measured if there is:
- known complicating preterm premature rupture of membranes
- known hourglass type membranes
- active vaginal bleeding
Treatment and prognosis
Management options can be controversial, with conflicting results, particularly regarding the efficacy of a cerclage placement as treatment. Bed-rest, tocolysis, cerclage (tracheloplasty), transabdominal suture placement, and administration of steroids to accelerate fetal lung maturity are all treatment options to be considered. A recent meta-analysis suggests that cerclage is effective in reducing preterm births by 26% in singleton pregnancies.
If the cervical length is <30 mm (<3 cm), close interval follow up is recommended.