Definition
Premature (preterm) labor refers to the onset of labor before 37–0/7 weeks’ gestation.
Incidence
- This is 8–12% (approximately one in eight) of all deliveries
- Accounts for 85% of all perinatal morbidity and mortality.
Pathophysiology
Preterm labor represents either a breakdown in the mechanisms responsible for maintaining uterine quiescence throughout pregnancy or a short-circuiting or overwhelming of the normal parturition cascade that triggers labor prematurely. Four discrete pathways are described, including stress, infection, stretch, and hemorrhage (Figure 57.3).
Etiology
- Preterm labor represents a syndrome rather than a diagnosis because the etiologies are varied (Figure 57.2).
- Of all preterm births, 20% are iatrogenic (performed for maternal or fetal indications), 30% are associated with intra-amniotic infection/inflammation, 20–25% are associated with preterm premature rupture of membranes (PPROM), and 20–25% result from spontaneous (idiopathic) preterm labor.
Prediction of preterm birth
- Risk factors for preterm birth have been identified (see Table 57.1). However, reliance on historic/demographic risk factors alone will fail to identify >50% of pregnancies that deliver preterm.
- Although an increase in uterine activity is a prerequisite for preterm labor, home uterine monitoring has not been shown to decrease the incidence of preterm birth.
- Serial cervical evaluation is reassuring if the examination remains normal. However, an abnormal finding (dilation or effacement) is associated with preterm delivery in only 4% of low-risk and 20% of high-risk women.
- There is an strong inverse correlation between sonographic cervical length (CL) and preterm delivery in both high- and low-risk pregnancies. CL is not currently recommended in low-risk patients. In high-risk pregnancies, baseline CL is recommended at 16–20 weeks followed by serial CL every 2–4 weeks until 30–32 weeks.
- A number of biochemical/endocrine markers have been associated with preterm delivery, but only cervicovaginal fetal fibronectin (fFN) has been established as a screening tool. The value of fFN lies in its negative predictive value: 99% of women with a negative fFN at 22–24 weeks will still be pregnant in 1 week, 98% in 2 weeks, and 89% in 3 weeks. However, the positive predictive value is poor (only 25% of women with a positive fFN will deliver before 35 weeks).
- Vaginal infections (bacterial vaginosis, Neisseria gonorrhoeae
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