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

Objectives To compare transabdominal and transvaginal sonographic measureme nts of cervical length in pregnancy and examine the factors that may influe nce these measurements. Methods The study population consisted of 149 women with singleton pregnanc ies attending for routine ultrasound examination at 23 weeks of gestation. In all women the cervix was successfully visualized by transvaginal sonogra phy and cervical length was measured after emptying of the bladder. This me asurement was compared to that obtained by transabdominal sonography. In ad dition the ability to visualize the cervix transabdominally was examined in relation to body mass index (BMI), bladder volume and cervical length. Results The percentage of cases in which the cervix could be seen transabdo minally increased from 42% for bladder volume of < 50 ml to 73% for volumes > 150 ml, and 13% tor cervical length <20 mm to 51% for lengths > 40 mm. T he 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 sig nificantly longer than those with an empty bladder. Conclusion The aim of cervical assessment in pregnancy rs to identity women with a short cervix because they are at high risk of preterm delivery. Thi s aim can not be fulfilled hv transabdominal sonography Such a scan fails t o visualize the cervix in a high proportion of cases and in particular thos e with a short cervix. Furthermore, successful visualization requires a ful l bladder which falsely increases cervical length.