Rate of change in cervical length in women with vaginal bleeding during pregnancy

To evaluate whether women with known risk factors for preterm birth will manifest different rates of cervical shortening preceding a spontaneous preterm birth.

We conducted a secondary analysis of data from the Maternal--Fetal Medicine Units Network Preterm Prediction Study. Known risk factors for preterm birth were recorded. Cervical lengths were measured between 22+0 weeks and 24+6 weeks, and again 4 weeks later. Cervical slope was defined as the change in cervical length between these visits divided by time (millimeters per week). Preterm birth was defined as preterm premature rupture of membranes or spontaneous preterm labor leading to delivery before 37 weeks of gestation. We analyzed the data for 2,584 women using logistic regression and tested for interaction between risk factors in the model to determine whether cervical shortening preceded preterm births in all variable groups.

Cervical slope was not significantly associated with preterm birth (P=.9) in women with vaginal bleeding. Cervical slope was significantly associated with preterm birth in women without a history of vaginal bleeding (odds ratio 1.2, 95% confidence interval 1.1-1.4).

Pregnancies without vaginal bleeding have a 20% increase in the risk of preterm birth for each additional millimeter per week increase in cervical slope. Pregnancies with vaginal bleeding are at risk for preterm birth but do not appear to undergo progressive cervical shortening. This suggests that women with vaginal bleeding undergo a different mechanism leading to preterm birth.

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