Short cervical length and cervical funneling found on transvaginal ultrasound examination at 24 to 28 weeks' gestation have been associated with delivery before 35 weeks' gestation. In addition, cervical changes before 20 weeks' gestation recently have been associated with preterm delivery. Andrews and colleagues studied the effectiveness of transvaginal ultrasonic measurements of the cervix before 20 weeks in predicting preterm delivery in high-risk women.

Women with a history of at least one spontaneous birth between 16 and 30 weeks' gestation were eligible for the study. Those with a history of multiple gestations, ruptured membranes, history of cervical cerclage, signs of preterm labor or serious medical problems were excluded. Women who met the study criteria were examined every two weeks by transvaginal ultrasound to measure the cervix, beginning at 15 weeks' gestation and continuing through 30 weeks' gestation. The findings were correlated with the extent of gestation at spontaneous labor. Spontaneous preterm birth was defined as a birth that occurred at less than 35 weeks.

Sixty-nine consecutive women were enrolled, and in all patients a Gram stain of vaginal secretions was obtained to identify possible bacterial vaginosis. Transvaginal cervical ultrasonography was performed on 53 women before 20 weeks' gestation. Mean cervical length progressively decreased with increasing gestational age. A short cervix was defined as one that was at or below 22 mm, the 10th percentile for the women in this study. Cervical funneling was associated with birth within two to four weeks and before 35 weeks' gestation. A short cervix and cervical funneling detected before 20 weeks' gestation were associated with spontaneous birth within four weeks and with delivery before 35 weeks' gestation. When detected between 20 and 29 weeks, these changes were associated with spontaneous delivery before 35 weeks' gestation but not with delivery within the four weeks following the study. The study data suggested that funneling might be a stronger indicator than cervical shortening.

The authors conclude that short cervical length and funneling of the internal os are associated with increased risk of spontaneous preterm birth. They suggest that transvaginal cervical ultrasonography performed early in pregnancy could identify candidates for intervention from among women with risk factors for preterm delivery. Among women with a history of at least one spontaneous preterm delivery, ultrasound examination before 20 weeks' gestation could identify those at greatest risk. However, the authors stop short of recommending specific clinical interventions until further studies are conducted to confirm the efficacy of these techniques and their role in designing interventions to prevent preterm deliveries.