Materials and Methods

We used data from the Centers for Disease Control and Prevention National Vital Statistics System’s period-linked birth and infant death files for 2007 and 2013. In this data set, information from death certificates for each person younger than 365 days old in a given year is linked to the birth certificate. Hence, information on the birth certificate, including maternal race and ethnicity and gestational age at birth, can be used to augment the death data, and these data comprise the numerator file.

The denominator file consists of all live births in a given year. In 2007 and 2013, 98.4% and 99.0% of infant deaths, respectively, could be linked to a corresponding birth certificate. The number of infant deaths in the linked file are weighted to equal the sum of the linked plus unlinked infant deaths by age at death and state, and these weights are applied during an analysis to account for the small fractions of unlinked infant deaths, thus resulting in counts representing the entire population.

The years 2007 and 2013 were chosen because 2007 is the first year that California, a state that has approximately 12% of births in the United States, reported gestational age at birth based on any criteria other than the last menstrual period, and 2007 was the beginning of the decline in the US infant mortality rate. The most recent year linked birth infant death data is 2013. A detailed description of the linkage can be found elsewhere. This public use data set is derived from deidentified birth certificates and death certificates and hence fall outside the definition of human subjects. Therefore, this analysis was not subject to institutional review.

We included all deaths and births for which there was a reported gestational age at birth on the birth certificate of 22 weeks or greater. Although gestational age based on the last menstrual period (LMP) recorded on the birth certificate has been the traditional source of gestational age in national statistics, a large body of research demonstrates the superiority of the obstetric estimate over the LMP-based estimate, and there appears to be little difference between the contemporary terms of obstetric estimate and clinical estimates.

In 2014, the National Center for Health Statistics began using the obstetric estimate as the preferred measure of gestational age for national reporting. The obstetric estimate is defined by the National Center for Health Statistics as “the best estimate of the infant’s gestation in completed weeks based on the birth attendant’s final estimate of gestation.” Hence, the first choice for gestational age in this analysis was based on the obstetric or clinical estimate (referred to hereafter as obstetric estimate). If the obstetric estimate of gestational age was missing and an LMP-based estimate was available, the LMP-based estimate was used as the estimate for gestational age.

In 2007, 465 infant deaths and 13,452 births (1.5% and 0.3% of deaths and births, respectively) and in 2013, 59 infant deaths and 3822 births (0.3% and 0.1% of deaths and births, respectively) had LMP-based gestational age estimates because of missing obstetric estimates and available LMP-based estimates.

For race- and ethnicity-specific analyses, maternal race and ethnicity was obtained from birth certificates and recorded as non-Hispanic black, non-Hispanic white, and Hispanic. Race and ethnicity from the birth certificate is considered more reliable than from the death certificate because they are reported by the mother, whereas the race and ethnicity of a decedent are reported by funeral directors, and there may be variability in the sources of that information.

Infant mortality was viewed as the product of the number of births at each gestational age (GA) and the GA-specific mortality. Hence, the total infant mortality rate (IMR) can be expressed as follows:

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