How is cervical incompetence evaluated sonographically?
Cervical length can first be assessed transabdominally with a partially distended bladder. A fully distended bladder may falsely lengthen the cervix due to mass effect.
If the cervix is not well visualized or appears shortened transabdominally, then transvaginal or transperineal imaging should be performed.
The normal cervical length is ≥3 cm. A length of 2 to 3 cm is considered borderline shortened, and <2 cm definitively shortened. If the internal os is open (referred to as funneling), the maximal width of the dilated cervical canal should also be measured, and any prolapse of membranes, umbilical cord, and/or fetal parts should be documented. The major risk of an incompetent cervix is preterm delivery.
