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for gestational age have implications for mortality and morbidity. Thus, many preterm infants who are large for gestational age have normal birth weights but have rates of mortality and morbidity different from those of full-term infants with normal birth weights. Few studies report outcomes by gestational age category.

The inaccuracy of data on gestational age is a major problem for research on preterm birth. Several methods are used to determine gestational age. Early prenatal ultrasounds (before 20 weeks of gestation) are more accurate than any other prenatal or postnatal estimate of pregnancy duration (Alexander et al., 1992; Chervenak et al., 1998; Nyberg et al., 2004). Despite its accuracy in estimating gestational age, the routine use of prenatal ultrasounds to estimate the duration of pregnancy is limited by several factors. Access to prenatal care is an issue for many of the women who are at the highest risk of preterm delivery (i.e., young, poor, and immigrant women); and there are racial disparities in prenatal care. In addition, the United States has no national standard for the routine use of prenatal ultrasound. Although more pregnant women in the United States are receiving ultrasounds than in the past (68 percent in 2002 versus 48 percent in 1989), many may be performed too late in pregnancy or the quality of the ultrasound may not be sufficient for the accurate and reliable estimation of the duration of pregnancy (Martin et al., 2003). Although early prenatal ultrasound is a good recommendation for obstetric practice, for research an earlier ultrasound (well within the first trimester) would be better, as it would allow investigation of potential factors that affect the growth trajectory earlier in pregnancy.

Although much attention has been paid to obtaining accurate obstetric estimates of gestational age, there is a similar need for more methods of assessing fetal and infant maturity. Maturity assessment is even more important when gestational age is unknown or uncertain. In lieu of functional measures of fetal or infant maturity, accurate measures of gestational age are essential for clinical care as well as research on the causes, mechanisms, and outcomes of preterm birth.

The National Center for Health Statistics develops standards for uniform reporting of live births and fetal, neonatal, and infant deaths to national public health databases. Although birth certificates are intended to establish the date of birth, citizenship, and nationality, they contain valuable public health information and are the only national source of birth weight and gestational age data. Large state and national population databases with birth and death certificate data have been used to plot gestational age distributions, birth weight for gestational age, and gestational age- and birth weight-specific rates of neonatal mortality. Gestational age is used to calculate a variety of statistical indicators used to monitor the health

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