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Preterm birth: Risk factors and interventions for risk reduction

Julian N Robinson, MD
Errol R Norwitz, MD, PhD, MBA
Section Editor
Charles J Lockwood, MD, MHCM
Deputy Editor
Vanessa A Barss, MD, FACOG


Preterm birth refers to a delivery that occurs before 37 weeks of gestation. It may or may not be preceded by preterm labor. Seventy to eighty percent of preterm births (PTBs) (table 1) are spontaneous: due to preterm labor (40 to 50 percent) or preterm premature rupture of membranes (20 to 30 percent); rarely cervical insufficiency results in spontaneous preterm birth. The remaining 20 to 30 percent of PTBS are iatrogenic: due to maternal or fetal issues that jeopardize the health of the mother or fetus (eg, preeclampsia, placenta previa, abruptio placenta, fetal growth restriction, multiple gestation). Complications of pregnancy can lead to both spontaneous and provider-initiated preterm births.

There are many risk factors for PTB (table 2) and many pathways from these risk factors to the terminal cascade of events resulting in labor. Preterm labor likely occurs when local uterine factors prematurely stimulate this cascade or suppressive factors that inhibit the cascade and maintain uterine quiescence are withdrawn prematurely. The four major factors leading to preterm labor are intrauterine infection, decidual hemorrhage, excessive uterine stretch, and maternal or fetal stress. Uteroplacental vascular insufficiency, exaggerated inflammatory response, hormonal factors, cervical insufficiency, and genetic predisposition also play a role. (See "Pathogenesis of spontaneous preterm birth".)

Ideally, identification of modifiable and nonmodifiable risk factors for PTB before conception or early in pregnancy will lead to interventions that help prevent this complication. However, few interventions have been proven to prolong pregnancy in women at risk. This goal has been elusive for several reasons: two-thirds of PTBs occur among women with no risk factors, causality has been difficult to prove (eg, a cofactor may be required thus complicating the chain of causality), and no adequate animal model exists for study of spontaneous PTB (sPTB).

Risk factors for PTB and potential interventions to mitigate risk, when possible, will be reviewed here. Pathogenesis of PTB and diagnosis and treatment of preterm labor are discussed separately:

(See "Pathogenesis of spontaneous preterm birth".)

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Literature review current through: Nov 2017. | This topic last updated: Oct 26, 2017.
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