A 20-year old, asymptomatic primigravid woman with a singleton pregnancy at 20w 3d gestation had a routine ultrasound examination to assess fetal anatomy. Although no structural fetal abnormalities were noted, the unexpected findings shown below were identified. The distance marked by the calipers was 20 mm.
The sonogram shows a shortened cervical length and funneling of the membranes into the upper portion of the cervical canal. Women with an incidentally-detected short cervical length (less than or equal to 20 mm) in the second trimester in the absence of a prior singleton preterm birth (PTB) should be treated with vaginal progesterone to reduce the risk of preterm birth. A transvaginal cerclage would be indicated if the cervix continues to shorten despite use of progesterone. The value of pessary placement for prevention of preterm delivery remains problematic.
Short cervical length on ultrasound examination in the second trimester is a well-documented risk factor for preterm delivery, as are uterine anomaly, low maternal weight, and low socioeconomic status. However, a history of prior preterm birth is the strongest risk factor.
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