Premature or preterm birth is birth less than 37 weeks gestation based on the obstetric estimate of gestational age. Data used in this report card came from the National Center for Health Statistics (NCHS) natality files, as compiled from data provided by the 57 vital statistics jurisdictions through the Vital Statistics Cooperative Program. This national data source was used so that data are comparable for each state and jurisdiction-specific report card. Data provided on the report card may differ from data obtained directly from state or local health departments and vital statistics agencies, due to timing of data submission and handling of missing data. The preterm birth rate shown at the top of the report card was calculated from the NCHS 2017 final natality data. Preterm birth rates in the trend graph are from the NCHS 2007-2017 final natality data. County preterm birth rates are from the NCHS 2016 final natality data. Preterm birth rates for bridged racial and ethnic categories were calculated from NCHS 2014-2016 final natality data. Preterm birth rates were calculated as the number of preterm births divided by the number of live births with known gestational age multiplied by 100.

Report cards for states and jurisdictions, except District of Columbia, display up to 6 counties with the greatest number of live births. Counties are not displayed if the number of preterm births is less than 20. Counties are ordered alphabetically. Grades were assigned based on criteria described above. Change from previous year was assessed by comparing the 2016 county preterm birth rate to the 2015 rate.

Mother's race and Hispanic ethnicity are reported separately on birth certificates. Rates for Hispanic women include all bridged racial categories (white, black, American Indian/Alaska Native, and Asian/Pacific Islander). Rates for non-Hispanic women are classified according to race. The Asian/Pacific Islander category includes Native Hawaiian. In order to provide stable rates, racial and ethnic groups are shown on the report card if the group had 20 or more preterm births in each year from 2010-2016. To calculate preterm birth rates on the report card, three years of data were aggregated (2014-2016). Preterm birth rates for not stated/unknown race are not shown on the report card.

The March of Dimes disparity ratio is based on Healthy People 2020 methodology and provides a measure of the differences, or disparities, in preterm birth rates across racial/ethnic groups within a geographic area.¹ The disparity ratio compares the racial/ethnic group with the lowest preterm birth rate (comparison group) to the average of the preterm birth rate for all other groups.

To calculate the disparity ratio, the 2014-2016 preterm birth rates for all groups (excluding the comparison group) were averaged and divided by the 2014-2016 comparison group preterm birth rate. The comparison group is the racial/ethnic group with the lowest six-year aggregate preterm birth rate (2010-2015) among groups that had 20 or more preterm births in each year from 2010-2015. A disparity ratio was calculated for U.S. states, the District of Columbia, and the total U.S. A disparity ratio was not calculated for New Hampshire, Maine, Puerto Rico, Vermont, and West Virginia. A lower disparity ratio is better, with a disparity ratio of 1 indicating no disparity. Progress toward eliminating racial and ethnic disparities was evaluated by comparing the 2014-2016 disparity ratio to a baseline (2010-2012) disparity ratio. Change between time periods was assessed for statistical significance at the 0.05 level using the approach recommended by Healthy People 2020.¹

If the disparity ratio significantly improved because the average preterm birth rate for all other groups got better, we displayed "Improved" on the report card. If the disparity ratio significantly worsened because the lowest group got better or the average of all other groups got worse, we displayed "Worsened" on the report card. If the disparity ratio did not significantly change, we displayed "No Improvement" on the report card.

The report card also provides the percent difference between the racial/ethnic group with the 2014-2016 highest preterm birth rate compared to the combined 2014-2016 preterm birth rate among women in all other racial/ethnic groups. This percent difference was calculated using only the racial/ethnic groups displayed on the state or jurisdiction-specific report card. This difference was calculated for each U.S. state with adequate numbers and the District of Columbia.

¹Talih M, Huang DT. Measuring progress toward target attainment and the elimination of health disparities in Healthy People 2020. Healthy People Statistical Notes, no 27. Hyattsville, MD: National Center for Health Statistics. 2016.

All calculations were conducted by the March of Dimes Perinatal Data Center.