Studies of larger preterm infants have not demonstrated a decline in the infant mortality rate in recent years.10,11,13 Fanaroff et al reported trends in morbidity and mortality rates for very-low-birth-weight infants (less than 1,500 g) born at one of 12 National Institute for Child Health and Human Development Neonatal Research Network centers from 1991 through 2002.10 Although both morbidity and mortality improved from the 1990–1991 infant cohort to the 1995–1996 cohort, no significant changes were noted between the 1995–1996 cohort and the 1997–2002 cohort. A recent National Center for Health Statistics report noted that the overall infant mortality rate did not change significantly between 2000 and 2005.11 This report also noted a lack of decline in mortality for very-low-birth-weight infants and for very preterm infants (born less than 32 weeks of gestational age) during the same time period. These studies did not present data that were gestational age-specific or data for only extremely preterm infants. The results of this study confirm the observation that the infant mortality rate specific for infants born less than 28 weeks of gestational age in the United States has reached a plateau.
The rising cesarean delivery rate for extremely preterm infants documented in this study is consistent with studies of larger preterm infants.18–20 The decision to perform a cesarean delivery at these gestations is complex because maternal, fetal, and neonatal morbidity and mortality risks must be considered. Because mode of delivery is influenced by fetal and maternal conditions,16,20–22 differences in the infant mortality rate between vaginal birth compared with cesarean delivery are not surprising.14,21,22 However, data on whether all extremely preterm infants would benefit from a cesarean delivery are unclear. Mortality rates for breech-presenting preterm infants seem to be lower after cesarean delivery compared with vaginal delivery,16,21,22 but results vary by gestational age and birth weight for vertex-presenting preterm infants.16,20–22 Cesarean delivery has been associated with a lower mortality rate for vertex preterm infants with a birthweight of 500–749 g in some,16,21 but not all,20 studies. Conflicting data has also been reported for vertex very-low-birth-weight preterm infants of larger birth weights20,21 and gestational age ranges,16,20,21 infants with fetal growth restriction,20,22 or those who are either small or appropriate size for gestational age.23 Recently, the World Association of Perinatal Medicine concluded “the available scientific evidence does not support a recommendation for Cesarean delivery for improving survival or decreasing morbidity for the extremely premature fetus.”24
The increase in the number of infants born extremely preterm demonstrated in this study is consistent with the rising preterm birth rate in general reported by others.11–13,19 This trend may be partially related to the increase in the rate of induced or operative deliveries, which has recently been reported for preterm infants.18,25,26 Currently only 51% of all preterm deliveries are attributable solely to spontaneous preterm labor.18 Unfortunately, this trend toward an increased rate of obstetric intervention has not been associated with obvious benefit in terms of an improved mortality rate for preterm infants.11,14,18,20 This study documented a similar increase in the cesarean delivery rate for extremely preterm infants without demonstrating an associated improvement in the infant mortality rate. In addition, this study demonstrated a significant rise in the infant mortality rate after the cesarean delivery of extremely preterm infants. These observations suggest that obstetric interventions may have resulted in a subtle shift toward a lower gestational age for the extremely preterm group in a manner similar to that of term infants.26 This has important implications because mortality rates increase dramatically as the gestational age at birth decreases and extending pregnancy even a few days may benefit the fetus.27
Gestational age-specific infant mortality rates remained steady, except at 24 weeks of gestational age. Because the decrease in mortality was primarily from 2004 to 2005, this may be a statistical outlier that does not persist. Alternatively, it may represent a change in approach to infants born at the “threshold of viability.” Recent approaches to resuscitation have been suggested, which are not based solely on the gestational age,28,29 and there is evidence physicians are becoming more aggressive with care.14,15,25 If sustained, the lower mortality rate at 24 weeks of gestational age may reflect a greater effort to maximize an infant's chances for survival at this gestational age.
Limitations to this study include the validity of gestational age assignment for a small percentage of infants,17 the inability to link 2–3% of infant deaths to their corresponding birth certificates (although the data are weighted up to the complete count of infant deaths in the United States),11,17 and potential misclassification of some fetal deaths as early infant deaths.30 These limitations are related to the nature of the data set but are unlikely to be significantly different between study years or mode of delivery. Because data regarding circumstances at the time of delivery were not available, the primary reason cesarean deliveries were performed is not known. Implications for clinical practice are limited because results are from nonrandomized population-based data. Study strengths include the large number of births and infant deaths, data specific to gestational age and the extremely preterm infant population, analysis of time trends, and the comprehensive records for the United States population provided by the National Center for Health Statistics.
In conclusion, the infant mortality rate for extremely preterm infants born at less than 28 weeks of gestational age in the United States from 1999 to 2005 did not improve despite a dramatic increase in the cesarean delivery rate. The accelerated rise in the number of infants being born extremely preterm and the increasing mortality rate after cesarean delivery for this population of infants are worrisome trends that warrant further investigation.
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