Pitfalls in ultrasonic cervical length measurement for predicting pre-termbirth

Objective: To describe the anatomic and technical difficulties encountered with transvaginal ultrasound imaging of the cervix in a consecutive series of women at risk for preterm delivery. Methods: Three groups of women had cervical ultrasound examinations: those with histories of preterm birth, those with incompetent cervices, and those admitted for preterm labor that did not progress. Standardized ultrasound examinations of the cervix involved measuring the length of the endocervica l canal, funneling length, and internal os dilation with and without fundal pressure. Results: Sixty consecutive women had transvaginal ultrasound examinations f or assessment of the cervix. Forty-six had histories of preterm birth, five had incompetent cervices, and nine had arrested preterm labor. Six types o f problems arose, which can be divided into anatomic or technical considera tions, with an overall frequency of 27% (95% confidence interval 16%, 40%). Anatomic pitfalls that hampered identification of the internal os included an undeveloped lower uterine segment (n = 5), a focal myometrial contracti on (n = 1), rapid and spontaneous cervical change (n = 1), and an endocervi cal polyp (n = 1). Technical pitfalls included incorrect interpretation of internal os dilation because of vaginal probe orientation (n = 7) and artif icial lengthening of the endocervical canal because of distortion of the ce rvix by the transducer (n = 1). Conclusion: We caution those who perform cervical length examinations to be wary of falsely reassuring findings due to potential anatomic and technica l pitfalls. (Obstet Gynecol 1999;93:510-6. (C) 1999 by The American College of Obstetricians and Gynecologists.).