Recurrence of spontaneous versus medically indicated preterm birth

Despite the increased tendency of preterm birth to recur, little is known with regard to recurrence risks for spontaneous and medically indicated preterm birth as well as recurrence risks in relation to severity of preterm birth. We examined the recurrence of spontaneous and medically indicated preterm birth.

A population-based, retrospective cohort study of births in Missouri (1989 to 1997) was carried out with analyses restricted to women who delivered their first 2 consecutive singleton live births (n = 154,809). Women who experienced spontaneous onset of labor and subsequently delivered preterm (less than 35 weeks) were classified as spontaneous preterm birth. Medically indicated preterm birth included women who delivered preterm through a labor induction or a prelabor cesarean delivery. Risk and odds ratio of preterm birth recurrence were derived from fitting multivariate conditional logistic regression models after adjusting for potential confounders.

If the first pregnancy resulted in a spontaneous preterm birth, then affected women were more likely to deliver preterm spontaneously (adjusted odds ratio 3.6, 95% confidence interval 3.2, 4.0) and also as a medically indicated preterm birth (odds ratio 2.5, 95% confidence interval 2.1, 3.0) in the second birth. Similarly, if the first pregnancy resulted in a medically indicated preterm birth, affected women were 10.6-fold (95% confidence interval 10.1, 12.4) more likely to deliver preterm because of medical indications in the second pregnancy as well as preterm spontaneously (odds ratio 1.6, 95% confidence interval 1.3, 2.1). The greatest risk of recurrence of preterm birth in the second pregnancy tended to occur around the same gestational age as preterm birth in the first pregnancy, regardless of the clinical subtype.

The observation that spontaneous preterm birth is not only associated with increased recurrence of spontaneous but also medically indicated preterm birth and vice versa, suggests that the 2 clinical subtypes may share common etiologies.