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0.73; 95% CI 0.65–0.82) compared with that among women with BMIs less than 30. Those investigators did not differentiate spontaneous from indicated preterm birth.

In a recent secondary analysis from the National Institute of Child Health and Human Development MFM Units Network Preterm Prediction study, in which 65 percent of the sample was African American, Hendler and colleagues (2005) found a decreased odds of spontaneous preterm birth at less than 37 weeks of gestation among women with prepregnancy BMIs greater than or equal to 30 (OR 0.57; 95% CI 0.39–0.83) compared with that among women with BMIs less than 30.

Even though obesity is detrimental for numerous aspects of human health and disease, high BMIs are associated with better outcomes of both congestive heart failure and atherosclerotic heart disease among people with chronic renal disease (Beddhu, 2004; Kalantar-Zadeh et al., 2004). It has been hypothesized that these epidemiological paradoxes may be the result of obesity-related changes in systemic inflammation (Beddhu, 2004; Kalantar-Zadeh et al., 2004).

Family History and Spontaneous Preterm Birth

Several observations support the hypothesis that spontaneous preterm birth is influenced by a family history of preterm birth. First, evidence from two studies performed with twins suggests a genetic predisposition for preterm birth with estimates of the proportion of preterm births among women with a family history of preterm birth ranging from 20 to 40 percent (Blackmore-Prince et al., 2000; Fuentes-Afflick and Hessol, 2000). Other observations support the idea that genetic factors affect the risk of preterm birth: (1) the leading risk factor for preterm birth is a previous preterm birth (James et al., 1999; Klerman et al., 1998; Shults et al., 1999); (2) an association between race-ethnicity and preterm birth persists in some instances, even if it is corrected for socioeconomic condition (Ekwo and Moawad, 1998); and (3) mothers who were preterm themselves (Basso et al., 1998) or who have a sister who had delivered an infant preterm (Kallan, 1997) have an increased risk of delivering their infants preterm.

Short Interpregnancy Interval and Preterm Birth

Interpregnancy interval is defined as that interval between the termination of one pregnancy and the conception of another. Numerous investigators have found a univariate association between short interpregnancy interval and a number of adverse perinatal outcomes, including preterm birth, low birth weight, and stillbirth (Adams et al., 1997; Al-Jasmi et al., 2002; Basso et al., 1998; Blackmore-Prince et al., 2000; Brody and Bracken, 1987; Conde-Agudelo et al., 2005; Dafopoulos et al., 2002; Ekwo and Moawad,

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