Objective To study longitudinal changes in the cervix during pregnancy using transvaginal ultrasound and secondly, to determine whether the measures used change with gestational age, and whether there are differences between nulliparous and primiparous women.

Design A longitudinal study involving five transvaginal ultrasound examinations of each woman at specified gestational ages from 18 to 30 weeks.

Participants Twenty-one nulliparous and 20 primiparous women completed the study and were delivered at term. Cervical length, diameter and dilatation were assessed at each examination.

Results In both nulliparous and primiparous women there is no significant change in either cervical length or diameter over the time period studied. In primiparous women the cervix is significantly longer than in nulliparous women (44.4 (5.1) mm versus 40.6 (4.7) mm [mean (SD)]; P 0.001). The cervical diameter in primiparous women is also significantly greater (31.8 (4.0) mm versus 29.0 (3.7) mm; P < 0.001).

Conclusions Cervical length and diameter are constant in both nulliparous and primiparous women throughout this gestational period. The cervix in primiparous women is both longer and thicker than in nulliparous women.