Description of the data source, numerator, denominator, survey questions, and other relevant details about the national estimate.
Estimates of preterm birth are calculated based on gestational age information reported on the birth certificate. The primary measure used to determine the gestational age of the newborn is the interval between the first day of the mother’s last normal menstrual period (LMP) and the date of birth, and presented as number of weeks. It is subject to error for several reasons, including imperfect maternal recall or misidentification of the LMP because of postconception bleeding, delayed ovulation, or intervening early miscarriage. These data are edited for LMP-based gestational ages that are clearly inconsistent with the infant’s plurality and birthweight, but reporting problems for this item persist and may occur more frequently among some subpopulations and among births with shorter gestations.
The 1989 revision of the U.S. Standard Certificate of Live Birth includes an item, “clinical estimate of gestation” and the 2003 revision of the birth certificate includes a comparable item “Obstetric estimate of gestation”. The clinical or obstetric estimate was compared with length of gestation computed using the LMP when the latter appears to be inconsistent with birth weight. This was done for normal weight births of apparently short gestations and very low birth weight births reported to be full term. For those births, the clinical or obstetric estimate was used if it was compatible with the reported birth weight. The clinical or gestation estimate was also used if the LMP date was not reported.
A description of the primary measurement used to determine the infant’s gestational age, the interval between the first day of LMP and the birth has been published by NCHS.
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