The rate of cervical change and the phenotype of spontaneous preterm birth.

Objective: Preterm birth is classified by the presence of uterine contractions and/or amniorrhexis at clinical presentation. This classification does not include prior cervical change. We hypothesized that the rate of cervical shortening before preterm birth would not differ according to clinical presentation.

Study Design: We analyzed data from a completed study of paired cervical ultrasound measurements to test our hypothesis. Cervical ultrasound measurements obtained 4 weeks apart in the second trimester were related to gestational age and clinical presentation at birth.

Results: Of 2521 eligible women, 128 were delivered after preterm labor and 106 after preterm membrane rupture; 89 delivered preterm for a medical or obstetrical indication; 2198 delivered at term. The rate of change was similar in women who presented with preterm labor (-0.96 mm/week) and preterm ruptured membranes (-0.82 mm/week).

Conclusion: Cervical shortening occurs at the same rate before spontaneous preterm birth, regardless of presentation.