Beginning in 2005, in recognition that neonates born between 34°/7 weeks and 366/7 weeks experience morbidities and mortality characteristic of premature newborns, preterm births were subdivided. Those before 336/ weeks are labeled earypreterm, and those occurring between 34 and 36 completed weeks are late preterm. Indeed, compared with births at weeks through 40617 weeks, these late-preterm infants experience morbidities that are also associated with prematurity (Spong, 2013). Recently, this concepthas expanded to births 37°/7 weeks through 386/ weeks, which are now defined as eary term, and those 39°/7 weeks through weeks, which are deined as term.
his revised terminology has led some to redefine a short gestation as those <39°/ weeks. By doing so, more than a third of live births in the United States in 2015 would be defined as having a shortened period of gestation (Martin, 2017). One implication is that only 65 percent of births in the United States occurred during the optimal 39 to 41 weeks' gestation. This emphasizes the realization that fetal maturation in humans is a continuum that is completed later in human pregnancy than previously appreciated. As a result, adverse neonatal sequelae from neonatal immaturity with elective delivery before 39 completed weeks are appreciable (Reddy, 2009; Tita, 2009).
This knowledge resulted in the development and application ofthe "39-week rule" to deter nonmedically indicated deliveries In the United States, the preterm birth rate rose slightly from 9.57 percent in 2014 to 9.63 percent for 2015 (Martin, 2017). This marks the irst rise in this percentage since 2007. Although concerning, some argue that the drop in preterm birth rates from 2007 to 2014 reflected systematic bias associated with changes in obstetrical dating (Frey, 2016).
Speciically, beginningwith the 2014 data year, the National Vital Statistics Reports from the National Center for Health Statistics transitioned to a new standard for estimating newborn gestational age for birth certificate completion (Martin, 2015). The new measure-obstetrical estimate of gestational age at delivery-replaced calculations based on the date of the last normal menses (Chap. 44, p. 846). As shown in Figure 42-1, these measures difer and do not provide equivalent absolute numerical comparisons ofpreterm birth rates. For example, the 2015 obstetrical estimate-based preterm birth rate was 9.6 percent compared with the last menstrual period-based rate of 11.3 percent (Martin, 2017). hus, current national data are now not directy comparable to previousy reported rates ofpreterm birth due to diering gestational age caculation methodologies. he national data are now reported starting with year 2007, which coincides with the year that this information became available.
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Stat Rep. 2015 Jun 1 ;64(5):1-20.) tem data showed that the overall reduction of preterm birth before 2014 was in part due to the changes in maternal age dis tribution. Speciically, teen birth rates declined. This translated into a drop in preterm birth rates over the same epoch and could explain the lower infant mortality rates (Callaghan, 2017).