Magnetic resonance imaging of three-dimensional cervical anatomy in the second and third trimester

Although a short cervix is known to be associated with preterm birth, the patterns of three-dimensional, anatomic changes leading to a short cervix are unknown. Our objective was to (1) construct three-dimensional anatomic models during normal pregnancy and (2) use the models to compare cervical anatomy in the second and third trimester.

A cross-sectional study was performed in a population of patients referred to magnetic resonance imaging (MRI) for a fetal indication. Using magnetic resonance images for guidance, three-dimensional solid models of the following anatomic structures were constructed: amniotic cavity, uterine wall, cervical stroma, cervical mucosa and anterior vaginal wall. To compare cervical anatomy in the second and third trimester, models were matched according the size of the bony pelvis.

Fourteen patients were imaged and divided into two groups according to gestational age: 20-24 weeks (n=7)) and 31-36 weeks (n=7). Compared to the second trimester, the third trimester was associated with significant descent of the amniotic sac (p=.02). Descent of the amniotic sac was associated with modified anatomy of the uterocervical junction. These three-dimensional changes were associated with a cervix that appeared shorter in the third trimester.

We report a technique for constructing MRI-based, three-dimensional anatomic models during pregnancy. Compared to the second trimester, the third trimester is associated with three-dimensional changes in the cervix and lower uterine segment.