Cervical assessment at the routine 23-weeks' scan: problems with transabdominal sonography

To compare transabdominal and transvaginal sonographic measurements of cervical length in pregnancy and examine the factors that may influence these measurements.

The study population consisted of 149 women with singleton pregnancies attending for routine ultrasound examination at 23 weeks of gestation. In all women the cervix was successfully visualized by transvaginal sonography and cervical length was measured after emptying of the bladder. This measurement was compared to that obtained by transabdominal sonography. In addition the ability to visualize the cervix transabdominally was examined in relation to body mass index (BMI), bladder volume and cervical length.

The percentage of cases in which the cervix could be seen transabdominally increased from 42% for bladder volume of < 50 ml to 73% for volumes > 150 ml, and 13% for cervical length < 20 mm to 51% for lengths > 40 mm. The ability to visualize the cervix was unrelated to BMI. Although there was a significant association between measurements taken transabdominally with those made transvaginally, measurements taken with a full bladder were significantly longer than those with an empty bladder.

The aim of cervical assessment in pregnancy is to identify women with a short cervix because they are at high risk of preterm delivery. This aim can not be fulfilled by transabdominal sonography. Such a scan fails to visualize the cervix in a high proportion of cases and in particular those with a short cervix. Furthermore, successful visualization requires a full bladder which falsely increases cervical length.