Cervical assessment at the routine 23-week scan: standardizing techniques

To examine the frequency of curvature of the cervix and to establish the degree of curvature with increasing cervical length. To assess the significance of this phenomenon in terms of the classification of patients as high risk for pre-term delivery.

Cervical length was measured prospectively by transvaginal sonography in 301 women at 23 weeks of gestation. The distance between the internal and external os was measured both as a straight line and also as a curved line along the endocervical canal. In addition, a search of our database was made to identify all women who had undergone cervical assessment as part of a policy of routine screening. In those with cervical length of less than 26 mm the thermal images of the cervix were used to obtain straight and curved measurements. The frequency of curved cervix was calculated and the relationship between the ratio of straight to curved measurement to the curved one was determined.

In the prospective study curvature of the cervix was observed in 143 (48%) of the 301 women. Curvature was observed in 51% (135 of 267) with cervical length of 26-55 mm, 25% (8 of 32) with length of 16-25 mm, and none of the two with length of 1-15 mm. The ratio of the straight to the curved measurement decreased with increasing cervical length (r = -0.27, P = 0.001). In the retrospective study curvature of the cervix was found in 72 (15%) of 471 with length of 16-25 mm and in none of the 76 with length of 1-15 mm.

Cervical length is influenced by the degree of cervical curvature. The disparity of measurements between the internal and external os taken as a straight line or along the cervical canal increases with cervical length. However, this disparity may not have any clinical implications because at short cervical length (less than 16 mm) the cervix appears to be always straight.