Incidence and complications
Preterm birth is defined as delivery occurring at less than 37 completed weeks’ gestation, and is a major cause of neonatal morbidity and mortality in developed countries. In the United States, preterm birth is the second leading cause of infant mortality (after congenital malformations). Moreover, the incidence of preterm birth has increased, from 9.4% in 1981 to 10.9% in 2005. This is due, at least in part, to rising rates of multiple gestations. In 1989, there were 2529 triplet gestations delivered in the United States, but 6800 cases in the year 2000. Infants born prematurely are at risk for respiratory distress syndrome (RDS), patent ductus arteriosus (PDA), intraventricular hemorrhage (IVH), necrotizing enterocolitis (NEC), and sepsis. They are also at risk for poor school performance, mental retardation, and growth delay. Cerebral palsy is a nonprogressive motor dysfunction disorder, which can have an onset near the time of delivery. Its incidence is 2:1000 births in the general population, but as high as 10% in infants born before 28 weeks’ gestation.
Risk factors for preterm birth
Risk factors for preterm birth include: preterm labor; prior preterm birth; multifetal gestation; second-trimester vaginal bleeding; low socio-economic status; familial history; chronic stress; poor nutrition; and chronic stress. However, more than 50% of preterm births occur in women with no identifiable risk factors. Furthermore, approximately one-third of preterm births are due to maternal or fetal complications (hypertensive disorders, placental abruption, placenta previa, multiple fetal pregnancy, congenital malformations), one-third is due to preterm premature rupture of membranes (PPROM), and one-third is idiopathic (with intact membranes).
Maternal infections outside the uterus (e.g. pneumonia, pyelonephritis, viral syndromes) are associated with an increased risk for preterm labor (PTL). Asymptomatic urinary tract infection with PTL is also associated with PTL.
Anatomic abnormalities of the uterus may account for up to 15% of PTL cases, with or without relative cervical insufficiency. Important congenital anomalies include septate and/or bicornuate uteri. Cervical insufficiency itself is recognized as a continuum and is responsible for at least some cases of preterm birth. Premature shortening and dilation of the cervix may result in exposure of the fetal membranes to bacteria. The anatomic and inflammatory components of preterm birth are thus often intermingled.
Congenital anomalies of the fetus, especially those associated with fetal hydrops or severe oligohydramnios, can also result in PTL. Uterine overdistension with severe polyhydramnios or multiple gestation is also associated with PTL. The high frequency of growth-restricted infants among those delivered preterm supports the association of placental insufficiency with PTL. Trauma is an uncommon but well-documented cause of PTL.
Subclinical genital tract infection leading to intra-amniotic infections by the ascending route has been shown to cause as much as 30% of spontaneous preterm birth and even a greater percentage of PPROM. Bacterial vaginosis and colonization with gonorrhea and chlamydia are associated with PTL and PPROM as well.
Diagnosis and treatment of preterm labor
The strict definition of PTL requires evidence of cervical change in response to regular uterine contractions. Nevertheless, in clinical practice therapy is commonly initiated on the basis of persistent contractions alone out of concern for difficulty in stopping advanced labor, which can lead to preterm birth. Interestingly, if the diagnosis is based solely on the presence of uterine contractions, studies have shown that up to 60% of patients are falsely diagnosed. In a patient without documented cervical change, a period of several hours of bedrest with monitoring may clarify the situation. The decision whether or not to initiate therapy can be guided with the addition of cervical length measurement via transvaginal ultrasonography (TVS) and/or assessment of fetal fibronectin (fFN) status. A patient with a cervical length ≥ 3.0 cm is very unlikely to deliver early. Similarly, a patient with a negative fFN has only a 3% or less chance of delivering within 2 weeks of the negative test.
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