A preterm birth is defined as a live birth occurring before 37 completed weeks of gestation. Births are categorized as preterm using national birth certificate data from the National Center for Health Statistics (NCHS). Gestational age is reported as the number of completed weeks of gestation on individual birth certificates and then categorized by NCHS. Preterm birth rate calculations are based on the number of live births to infants in a specific gestational age category divided by all live births excluding those missing data on gestational age, multiplied by 100. Overall, singleton-, and multiple-specific rates are calculated in the same way.
Beginning with 2014 data, the primary measure used by NCHS to determine the gestational age of the newborn is the obstetric estimate of gestation (OE). This measure replaces the previous measure based on the interval between the date of the mother's last menstrual period (LMP) and the infant's date of birth. The OE has been shown to more accurately reflect the true gestational age of the infant than LMP. NCHS edits OE data to include only gestational ages between 17 and 47 completed weeks. In 2014, 0.1% of records were either outside of this range or were missing information.
Calculations using OE gestational age differ from calculations using LMP gestational age. For example, in 2014, the US preterm birth rate based on OE was 9.6% of live births compared to 11.3% based on LMP. Despite these differences, both measures indicate a decline in the US preterm birth rate between 2007 and 2014.
In additional to overall preterm births, three subcategories of preterm birth are displayed on PeriStats: very preterm (less than 32 completed weeks); moderately preterm (32-36 completed weeks); and late preterm (34-36 completed weeks)
As with other indicators, preterm birth rates on PeriStats may differ from those provided directly by state health departments. This may be due to differences in the way the health department and NCHS calculate the gestational age of the infant or in the handling of missing data.
Additional information on the transition to OE gestational age and differences between OE and LMP gestational age measure can be found in:
Martin JA, Osterman MJK, Kirmeyer SE, Gregory ECW. Measuring gestational age in vital statistics data: Transitioning to the obstetric estimate. National vital statistics reports; vol 64 no 5. Hyattsville, MD: National Center for Health Statistics. 2015.
Hamilton BE, Martin JA, Osterman MJK, et al. Births: Final data for 2014. National vital statistics reports; vol 64 no 12. Hyattsville, MD: National Center for Health Statistics. 2015.
Martin JA, Wilson EC, Osterman MJ, Saadi EW, Sutton SR, Hamilton BE. Assessing the quality of medical and health data from the 2003 birth certificate revision: results from two states. National Vital Statistics Report. 2013 Jul 22;62(2):1-19.
Dietz PM, Bombard JM, Hutchings YL, Gauthier JP, Gambatese MA, Ko JY, Martin JA, Callaghan WM. Validation of obstetric estimate of gestational age on US birth certificates. American Journal of Obstetrics &Gynecology. 2014 Apr;210(4):335.e1-5. doi: 10.1016/j.ajog.2013.10.875.
Ambrose CS, Caspard H, Rizzo C, Stepka EC, Keenan G. Standard methods based on last menstrual period dates misclassify and overestimate US preterm births. Journal of Perinatalogy. 2015 April 2:35:411-414.