Is the Relationship between Short Interpregnancy Interval and Preterm Birth Related to a Short Cervix?

Our objective was to determine if the relationship between interpregnancy interval (IPI) and preterm delivery was related to cervical shortening.  This is a secondary analysis of a prospective cohort study designed to assess the relationship between cervical length (CL) and preterm delivery. Multiparous patients, who had transvaginal CLs obtained at 22 to 24 weeks and complete pregnancy outcome data available were included. The women were divided into two groups: Group 1 with an IPI of ≤ 1 year and Group 2 with an IPI of > 1 year. The primary outcome was short cervix (CL < 2.5 cm) at 24 weeks. The secondary outcome was the incidence of preterm delivery < 37 weeks. Interaction between short cervix and IPI was examined in a regression model.  There were 155 women in Group 1 and 328 women in Group 2. There was no difference in the incidence of short cervix between groups (10.0% in Group 1 and 12.9% in Group 2,  = 0.35). In a logistic regression model, short IPI and short cervix were independently associated with preterm birth.  The relationship between short IPI and preterm birth is not explained by a short cervix. They remain independent risk factors for preterm birth.