General Obstetrics and Gynecology: Obstetrics
Maternal-fetal conditions necessitating a medical intervention resulting in preterm birth

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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.

Section snippets

Material and methods

We utilized the state of Missouri's live birth data files from 1989 to 1997 for analysis. These data correspond to births abstracted from birth certificates of live-born infants.17, 18 Gestational age, reported in completed weeks, was largely based on menstrual dating. When the menstrual estimate of gestational age was inconsistent with the birth weight (eg, extremely low birth weight at term), a clinical estimate of gestational age, also contained on the vital records, was used instead, in

Results

The preterm birth rate at less than 35 weeks was 4.6% (n = 31,238). The distribution of maternal characteristics among near-term/term births and preterm births at less than 35 weeks is shown in Table I. Proportions of women aged less than 20 years, women with less than 12 years of schooling, Black race/ethnicity, single and those that smoked were higher in the preterm than near-term/term births.

Among all preterm births at less than 35 weeks, 23.5% (n = 7347) were medically indicated. The

Comment

The concept of separating preterm births on the basis of their underlying clinical subtypes to understand etiologies is not new.10, 11, 12, 23, 24 It has long been recognized that preterm birth is a heterogeneous end point with two major clinical subtypes, namely spontaneous and medically indicated preterm birth.3, 10, 25, 26, 27, 28 Much of the attention has been focused on its origins as well as the prediction and prevention of spontaneous preterm birth. However, 20% to 35% of all preterm

Acknowledgments

We thank the staff of the Missouri Vital Statistics Department for making the data files available to us. We appreciate the thoughtful comments from Darios Getahun, MD, MPH; Wendy Kinzler, MD; Yinka Oyelese, MD; and Hamisu Salihu, MD, PhD, on an earlier draft of the manuscript.

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    Supported in part through a grant (R01-HD038902) from the National Institutes of Health (to C.V.A.).

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