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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


Most preterm births (PTBs) are spontaneous: related to preterm labor or preterm premature rupture of membranes. The remainder are iatrogenic: performed because of medical or obstetrical complications that jeopardize the health of the mother or fetus.

There are many risk factors for PTB (table 1) 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prematurely stimulate this cascade or suppressive factors that inhibit the cascade and maintain uterine quiescence are withdrawn prematurely.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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