Objective: The objective of the study was to evaluate the extent to which maternal and fetal conditions necessitate medically indicated preterm birth.
Study design: A population-based, retrospective, cohort study of women who delivered a singleton live birth at 20 weeks or longer in Missouri, 1989 to 1997 was performed (n = 684,711). Maternal-fetal conditions that necessitated iatrogenic preterm birth included preeclampsia, small-for-gestational-age birth, fetal distress, placental abruption, placenta previa, unexplained vaginal bleeding, pregestational and gestational diabetes, renal disease, Rh sensitization, and congenital malformations. We examined the association between each of the aforementioned conditions and risk of medically indicated preterm birth at less than 35 weeks. Medically indicated preterm birth was defined as a labor induction or a prelabor cesarean in the absence of premature rupture of membranes at preterm gestations. Adjusted relative risk with 95% confidence interval for preterm birth was derived from multivariable logistic regression models, and population attributable fractions were calculated.
Results: The preterm birth rate (less than 35 weeks) was 4.6% (n = 31,238), with 23.5% (n = 7,347) of such births being medically indicated. Preeclampsia, fetal distress, small-for-gestational-age, and placental abruption were the most common indications for a medical intervention resulting in preterm birth, with at least 1 of these conditions present in 53.2% of medically indicated preterm births and in 17.7% of term births (relative risk 4.9, 95% confidence interval 4.7, 5.2).
Conclusion: Preeclampsia, fetal distress, small-for-gestational-age, and placental abruption, conditions that are associated with ischemic placental disease, are implicated in well over half of all medically indicated preterm births. Although the etiology of preterm birth is heterogeneous, it is reasonable that ischemic placental disease may serve as an important pathway to preterm birth.