birth permits identification of a group of women at the highest risk for whom an intervention may be tested and for whom intervention is most needed. The third motivation for prediction of spontaneous preterm birth is a corollary of the second: by identifying women at low risk for preterm birth, unnecessary, costly, and sometime hazardous interventions might be avoided. To date, no single test or sequence of tests has an optimal sensitivity or predictive value. This section reviews clinical, biophysical, and biochemical tests that can be used as predictors for preterm birth.

Clinical risk factors alone or in combination most frequently report a sensitivity of about 25 percent for prediction of preterm birth (Goldenberg et al., 1998; Mercer et al., 1996). Low prepregnancy weight (body mass index less than 19.8), genitourinary bacterial colonization or infection, and African American ethnicity have relative risks (RRs) of about twofold but contribute significant attributable risk because of their prevalence in the population. African American women deliver before 37 weeks of gestation twice as often as women of other races and ethnicities and deliver before 32 weeks of gestation three times as often as white women. The strongest risk factors in all racial-ethnic groups are multiple gestation (RR = five- to six-fold), a history of preterm birth (RR = three- to fourfold), and vaginal bleeding (RR = threefold).

The risk of preterm and low-birth-weight delivery rises in direct proportion to the number of fetuses, as can be seen Table 9-1.

Vaginal bleeding in pregnancy is a risk factor for preterm birth because of placenta previa, because of placental abruption, and when the origin is unclear (Ekwo et al., 1992; Meis et al., 1995; Yang et al., 2004b). Unexplained vaginal bleeding is particularly associated with preterm birth if it is persistent and if it occurs in white women (Yang et al., 2004b).

The risk of recurrent preterm birth rises with the number of prior preterm births, with maternal African American ethnicity, and as the gestational age of the prior preterm birth decreases (Adams et al., 2000; Mercer et al., 1999). The effect of a woman’s prior obstetrical history on the risk of preterm birth is shown in Table 9-2.

The data in Table 9-2 describe a homogeneous population from Norway. Data from the United States show the same phenomenon, with markedly increased rates of preterm birth for African Americans, reaching 50 percent or more for an African American woman with two or more prior preterm deliveries (Adams et al., 2000; Mercer et al., 1996). Other reported risk factors include the use of assisted reproductive technology, poor nutrition, periodontal disease, absent or inadequate prenatal care, age less than 18 years or over 35 years, strenuous work, high levels of personal stress,