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Obstetrics & Gynecology:
doi: 10.1097/AOG.0b013e318221001c
Original Research

Extremely Preterm Infant Mortality Rates and Cesarean Deliveries in the United States

Batton, Beau MD; Burnett, Christopher; Verhulst, Steven PhD; Batton, Daniel MD

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Author Information

From the Departments of Pediatrics and Statistics and Research Consulting, Southern Illinois University School of Medicine, Springfield, Illinois.

Presented in part at the annual Pediatric Academic Society Meeting, May 1–4, 2010, Vancouver, British Columbia, Canada.

Corresponding author: Beau Batton, MD, Southern Illinois University School of Medicine, Department of Pediatrics, Division of Neonatology, PO Box 19676, Springfield, IL 62794; e-mail: bbatton@siumed.edu.

Financial Disclosure The authors did not report any potential conflicts of interest.

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Abstract

OBJECTIVES: To estimate trends in infant mortality rates and cesarean delivery rates for extremely preterm infants born in the United States.

METHODS: This national population-based study used public data from the Centers for Disease Control and Prevention to investigate extremely preterm infants born alive between 22 0/7 and 27 6/7 weeks of gestational age from 1999 to 2005.

RESULTS: There were 177,552 extremely preterm infant births (fewer than 1% of all births) from 1999 to 2005. The number of annual extremely preterm births increased by 7% compared with a 4.5% increase for births at all gestations. During the study years, the extremely preterm infant mortality rate (percentage of infants who died in the first year) remained steady (range 33–34%; P=.22), whereas the cesarean delivery rate increased from 43% to 54% (P<.001). The infant mortality rate after cesarean delivery increased from 24% to 26% (P=.012). At each gestational age, the annual cesarean delivery rate increased over time (P<.001 for each), whereas gestational age-specific infant mortality rates were unchanged except for a 2% decline from 2004 to 2005 for infants born at 24 weeks of gestation (P=.01).

CONCLUSION: A significant rise in the cesarean delivery rate in the United States from 1999 to 2005 for infants born at less than 28 weeks of gestation was not associated with an improvement in the infant mortality rate.

LEVEL OF EVIDENCE: III

Infant mortality rates in the United States steadily declined in the latter half of the 20th century, particularly for preterm infants.1–3 These remarkable improvements in survival have commonly been attributed to the institution of regionalized perinatal care, the evolution of neonatal intensive care units, the increased use of prenatal steroids, and the development of surfactant therapy.4–9 More recently, the mortality rate for preterm infants seems to have reached a plateau because there has been little change in the last two decades.10–12 During this same time period, the preterm birth rate has steadily increased.11–13 This is partially related to an increased willingness on the part of physicians to perform a cesarean delivery for fetal indications at an increasingly lower gestational age.14–16 Although extremely preterm infants born at less than 28 weeks of gestational age are the least likely infants to survive, contemporary mortality rates and cesarean delivery rates in the United States specific to this population have not been well documented. Therefore, the objectives of this study were twofold: 1) to estimate recent trends in the extremely preterm infant mortality rate in the United States and; 2) to document the current cesarean delivery rate for the same population of infants.

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MATERIALS AND METHODS

Data were analyzed from the Centers for Disease Control and Prevention National Center for Health Statistics Linked Birth/Infant Death Records (www.wonder.cdc.gov/lbd) for all extremely preterm infants delivered between 22 0/7 and 27 6/7 weeks of gestational age who were born alive between January 1, 1999, and December 31, 2005 (the most recent year for which public data were available). During this time, the National Center for Health Statistics produced linked data in both birth cohort and period format. Period format data were used for this analysis. For example, the numerator for the 2005 period linked data represents all infant deaths occurring in 2005 (regardless of whether the infant was born in 2004 or 2005) and the denominator represents all infant births in 2005. Mode of delivery, gestational age, and record of death were compiled from birth certificates for all infants born alive in the United States and death certificates for all infants who died before their first birthday. These certificates are linked by corresponding numbers. During the study years, births in the United States were recorded using either the 1989 revised birth certificate or the 2003 revised birth certificate. Transition from the 1989 to the 2003 birth certificate occurred during 2004 and 2005. The 1989 revised birth certificate recorded the “clinical estimate of gestational age” and the 2003 revised birth certificate recorded the “obstetrician's estimate of gestation.” Standard practice in the United States during this time was to determine the gestational age based on the mother's last menstrual period and to revise it based on the obstetrician's examination or early prenatal ultrasound examination if indicated. When early ultrasonography is not performed and either the last menstrual period is uncertain or the last menstrual period-based gestational age is inconsistent with the infant's birth weight or physical examination, then gestational age is determined based on examination of the infant. No information on early prenatal ultrasound examination is recorded on birth certificates, but in the National Center for Health Statistics database, examination of the infant was used to estimate gestational age for 4.6–5.9% of all births annually from 1999 to 2005.17 The mortality rate was defined as the percentage of extremely preterm infants born alive who died in the first year.

Statistical analysis was performed using SAS 9.3 software. The chi-square test for linear trend was used to assess changes in mortality and cesarean delivery rates over time. The chi-square test was used to assess differences in mortality rates according to mode of delivery. This study was found to be exempt from institutional review board approval by the Springfield Committee for Research Involving Human Subjects at the Southern Illinois University School of Medicine.

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RESULTS

Of the 28,406,795 infants born during the study period, 177,552 (less than 1%) were born at 22 0/7 to 27 6/7 weeks of gestational age (Table 1). The annual number of births at all gestations increased by 4.5% (95% confidence interval, 4.48–4.52%) during the study period from 3,959,417 in 1999 to 4,138,573 in 2005 compared with a 7% (95% confidence interval, 6.68–7.32%) increase in the annual number of extremely preterm births from 24,893 in 1999 to 26,663 in 2005. The annual cesarean delivery rate for extremely preterm infants increased significantly during the study period from 43% in 1999 to 54% in 2005 (P<.001; Fig. 1), but the annual infant mortality rate did not change (range 33–34%; P=.22). The annual mortality rate for all extremely preterm infants was significantly lower after cesarean delivery compared with vaginal delivery (25% compared with 42%, P<.001). However, the annual infant mortality rate after cesarean delivery increased significantly with time from 24% in 1999 to 26% in 2005 (P=.012), whereas the infant mortality rate after vaginal delivery did not change.

Table 1
Table 1
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Fig. 1
Fig. 1
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The number of births at each gestational age during the study period increased with advancing gestational age from 16,170 at 22 weeks of gestational age to 42,006 at 27 weeks of gestational age. The annual cesarean delivery rate increased significantly over time at each gestational age (P<.001 at each gestational age; Fig. 2). The annual infant mortality rate at 22, 23, 25, 26, and 27 weeks of gestational age did not change during the study years (P>.1 at each GA; Fig. 2). The infant mortality rate at 24 weeks of gestational age decreased from 44% in 1999 to 41% in 2005 (P=.01), primarily because of a decrease from 43% to 41% between 2004 and 2005. From 1999 to 2004, the infant mortality rate at 24 weeks of gestational age did not change significantly (44% to 43%, P=.76). Gestational age-specific infant mortality rates from 22 to 26 weeks differed by mode of delivery with significantly higher rates noted after vaginal birth compared with cesarean delivery (Table 2). The infant mortality rate did not vary with mode of delivery at 27 weeks of gestational age.

Table 2
Table 2
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Fig. 2
Fig. 2
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DISCUSSION

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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Figure. No caption available.


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