Term infants are not all the same—mortality by gestational age

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Pediatricians are used to relating infant morbidities and mortalities to gestational age, as was recently modeled by Tyson et al for very low birth weight preterm infants (NEJM. 2008; 358: 1672-81). However, term infants (37-41 weeks) are generally considered as a homogenous group relative to morbidity and mortality outcomes. In recent years, the median gestational age at delivery in the US has decreased by about 1 week because of changing obstetric practices and maternal expectations. This shift has included increased numbers of late-preterm deliveries, with their associated risks. These risks have been highlighted in The Journal by both research reports and editorials over the last several years. However, the distribution curve for gestational age at delivery also includes increased numbers of infants that are delivered at 37 and 38 weeks, rather than 39 and 40 weeks. Although these infants are term by gestational age, Zhang et al asked if there were differences in morbidities and mortalities over the gestational age range of term deliveries. Using US national birth cohort data for 1995-2001, they found that mortality decreased from 37 to 39 weeks, was stable at 39 and 40 weeks, and then increased at 41 weeks. Lung related morbidities decreased, while meconium aspiration syndrome and birth injuries increased as gestation advanced. Deliveries earlier in the “term interval” may have adverse health outcomes that parallel, but are of lower frequency than those reported for late-preterm deliveries.