Three dimensional volume measurement of the cervix during pregnancy compared to conventional 2D-sonography

To compare the three dimensional (3D) volume assessment of the cervix with the conventional two-dimensional cervical length measurement in a low and a high risk group for cervical incompetence.

In an observational study, we investigated a group A of low risk pregnancies (no preterm contractions, no vaginal bleeding or vaginal infections and no history of preterm delivery) and a group B of high risk pregnancies (preterm contractions or PROM). All patients underwent a transvaginal ultrasound investigation with a 7.5 MHz probe using a three-dimensional ultrasound system (Combison 530, Kretztechnik, Austria). After measuring the cervical length, the internal os and the funneling with the B-mode, 3D-volume was recorded twice by the same investigator using the same machine. Since 2D-length measurement of the cervix has been established to be predictive for spontaneous preterm delivery, we wanted to test whether 3D-volume assessment has a better discriminative power to differentiate a high-risk from a low-risk group. Therefore the 2D and 3D measurements (mm, resp. cm3) were compared between patient groups A and B using the two tailed Student t-test and Fisher's Exact test.

In 2D cervical length measurement the mean cervical length in group A was significantly longer than in group B: 41.1 +/- 8.61 mm and 27.77 +/- 10.42 mm, P = 0.00000017. In 3D-sonography the mean cervical volume was larger in group A, but the difference compared to group B was not significant: 47.71 +/- 18.38 mm and 39.90 +/- 12.57 mm, P = 0.07.

Contrary to our hypothesis cervical length measurement therefore was superior to cervical volume measurement assessed by 3D ultrasound for identifying women with increased risk of spontaneous preterm delivery. This may be due to the larger distribution of measurement values in the 3D group.