Differences in birth weight for gestational age distributions according to the measures used to assign gestational age.

Population-based standards for infant size for gestational age depend on accurate assessments of birth weight and gestational age; the accuracy of the latter measure has been questioned. The authors sought to explore how different methods of assigning gestational age in vital records data affect distributions of birth weight for gestational age. The 2005 US natality file was used to create 4 measures of gestational age for singleton births consisting of measures found on the 1989 (last menstrual period (LMP) and clinical estimate) and 2003 (LMP and obstetric estimate) revisions of the US standard birth certificate: clinical or obstetric estimate and LMP-based estimate agree within 7 days ("gold standard"); clinical estimate only; obstetric estimate only; and LMP-based estimate only. Birth weight for gestational age distributions differed according to the measurement of gestational age. Regardless of birth certificate revision, the median, 10th, and 90th percentile distributions were virtually identical for the gold standard, clinical estimate, and obstetric estimate. Birth weights for the LMP estimate were higher for preterm births and lower for postterm births for both birth certificate revisions. Agreement between the gold standard estimate and clinical and obstetric estimates of gestational age suggests that using the LMP-based estimate for establishing norms should be revisited.