Transperineal versus transvaginal sonographic cervical length measurement in second- and third-trimester pregnancies

To investigate the reliability of, and patient satisfaction with, transperineal cervical length measurement during the third trimester of pregnancy and to compare these with measurement during the mid-trimester, using transvaginal sonographic imaging of the cervix as a reference.

Women attending an outpatient clinic for cervical length measurement before 29 weeks or after 35 weeks of gestation were examined by transvaginal and transperineal ultrasound and the results compared. Preference for either method was indicated by both sonographers and patients.

Seventy-one patients participated in the study, 23 in the second and 48 in the third trimester of pregnancy. There was failure to obtain a clear image on transperineal ultrasound in 30% of mid-trimester pregnancies, and in 19% of third-trimester cases. Elevation of the patient's hips improved the image in five out of 10 women in whom the scan was repeated following a postural change. Transvaginal cervical length measurements could be obtained in all cases. There was a strong correlation between transvaginal and transperineal measured cervical length (Pearson's correlation coefficient = 0.85). Sonographers preferred transvaginal images of the cervix irrespective of whether they were obtained in the second or third trimester. Transperineal ultrasound was judged as not or mildly painful by most women, but transvaginal ultrasound was preferred.

Transvaginal ultrasound is the least painful, most feasible, and probably most accurate method with which to measure cervical length in the third trimester of pregnancy. Transperineal ultrasound is a feasible alternative, although its application in clinical practice is restricted by the need for an experienced sonographer.