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The available literature has provided some evidence of familial and intergenerational influences on low birth weight or preterm birth (Bakketeig et al., 1979; Carr-Hill and Hall, 1985; Khoury and Cohen, 1987; Porter et al., 1997; Varner and Esplin, 2005). A population-based cohort study of data from birth certificates and fetal death certificates from the state of Georgia between 1980 and 1995 suggest that the recurrence of preterm delivery contributes to a notable portion of all preterm births, especially for the shortest gestations (Adams et al., 2000). Analysis of the data from the live birth cohort of the 1988 U.S. National Maternal and Infant Health Survey demonstrated a strong familial aggregation of low birth weight and preterm birth in both white and African American populations (Wang et al., 1995).

Familial and intergenerational influences on preterm birth may be attributable to shared environmental factors or genetic factors, or both. Studies with twins are a powerful approach to detecting the environmental and genetic components of a given disease or trait, but very few studies of preterm birth in human twins have been performed. The heritability of preterm birth was found to be 17 to 27 percent in an Australian population (Treloar et al., 2000), the heritability of gestational length was found to be 25 to 40 percent in a Swedish population (Clausson et al., 2000). The limited number of twin studies was in part due to the difficulty of assembling such a study population (i.e., female twins and their babies). The large registries of twins have the potential of probing not only concordance of preterm birth among female twins, but also the contributions of males and intergenerational effects.

As detailed below, with recent advances in human genetics and molecular biology, assessment of the contributions of genetics to human diseases has progressed from indirect measurements based on family history to direct measures of an individual’s genotype (genome sequence) at particular gene loci. Nevertheless, it should be emphasized that family history and a woman’s past medical history remain valuable tools in assessments of a woman’s risk for preterm birth.

Genetic Association Studies

Disorders found to be associated with changes in the sequence of a single gene have been associated with an increased risk of preterm birth, often as a result of a predisposition to polyhydramnios in pregnancies with fetuses with changes in the sequence of that single gene. Among these conditions are myotonic dystrophy, Ehlers-Danlos syndrome, Smith-Lemli-Opitz syndrome, and neurofibromatosis. However, like many other com-

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