About the Data
Description of the data source, numerator, denominator, survey questions, and other relevant details about the national estimate.
Number of infants born at less than 32 completed weeks of gestation
Number of live births
The Centers for Disease Control and Prevention’s National Center for Health Statistics (NCHS) has measured vital statistics gestational age data based primarily on the difference between the date of the last normal menses (LMP) and the date of infant’s birth since national LMP data first became available in 1981. However, the quality of LMP-based data has long been of concern. Imperfect maternal recall, misinterpretation of bleeding early in pregnancy, irregular menstrual cycles, and data entry errors have been shown to result in the misclassification of gestational age, particularly at preterm (under 37 completed weeks) and postterm (42 weeks and over).
An alternative measure of gestational age, the clinical estimate (CE), was added to the 1989 U.S. Standard Certificate of Live Birth. Detailed definitions and instructions for the new measure were not developed or released, however. Concerns with data quality and the lack of national reporting (California did not report the CE) precluded the estimate from being used as a national measure of gestational age. The CE was replaced with the similar item, the ‘‘obstetric estimate of gestation at delivery’’ (OE) with the 2003 birth certificate revision. More detailed definitions and instructions were developed and distributed for the OE, which in brief is defined as ‘‘the best estimate of the infant’s gestation in completed weeks based on the birth attendant’s final estimate of gestation’’. Despite differences in definitions and instructions, data for the CE and OE appear comparable and are combined in natality public-use files. National data for a combined OE-CE item did not become available until the 2007 data year, however.
Compared with LMP-based estimates, recent studies suggest higher consistency between OE-CE-based estimates and birthweight and better agreement between the OE-CE-based estimates and estimates of gestational age based on an early ultrasound (considered the gold standard). Agreement was also closer between the OE-CE estimates and gestational ages for births conceived using assisted reproductive technology, for which dates of conception were well documented. Studies also indicate high to moderate agreement between OE reporting on the birth certificate and information on best estimates of gestational age and estimated delivery dates on hospital medical records.
Increasing evidence of the greater validity of OE-based data compared with LMP-based data, and the national availability of OE data, have prompted NCHS to transition to the use of the OE as its standard, primary measure of gestational age.
Data for the obstetric estimate measure are based primarily on the 2003 U.S. Standard Certificate of Live Birth item ‘‘Obstetric estimate of gestation.’’ The obstetric estimate of gestation is defined as ‘‘the best obstetric estimate of the infant’s gestation in completed weeks based on the birth attendant’s final estimate of gestation’’.
Data for the remaining states are based on the 1989 revision of the U.S. Standard Certificate of Live Birth item ‘‘Clinical estimate of gestation.’’ The instructions to hospitals for the 1989 revision simply state that the birth attendant should provide a clinical estimate of gestation not based on the date of LMP and the date of birth. Despite differences in terminology and instructions, studies and NCHS’ own internal review of CE and OE data for the study period (available upon request; e-mail firstname.lastname@example.org) suggest that estimates based on the obstetric estimate and the clinical estimate of gestation are comparable. Accordingly, data for these two measures are combined for and are referred to as the OE.
The following report outlines how gestational age is measured in vital statistics data using LMP and the transition to the OE.
Any change to the objective text, baseline, target, target-setting method or data source since the Healthy People 2020 launch.
Additional resources about the objective
- National Center for Health Statistics. User guide to the 2010 natality public use file. Hyattsville, MD.