A study by University of Alabama at Birmingham (UAB) researchers published April 1, 2013, in the journal Pediatrics showed that increasing numbers of premature and other low birth weight infants are the leading cause for the leveling off of infant mortality and neonatal mortality rates in the United States.
Infant mortality rate is defined as the number of infants who die before their first birthday. Neonatal mortality rate is defined as the number of infants who die before reaching 28 days old.
Researchers concluded the leveling off of these rates is due to increases in live birth registration of smaller and more immature infants, particularly infants with birth weights of less than 500 grams or 1.1 pounds.
"Doctors today are able to keep smaller babies alive due to improved obstetrical and neonatal care," said UAB Edwin M. Dixon Professor of Pediatrics Wally A. Carlo, M.D., the study's senior author, director of the UAB Division of Neonatology and director of UAB's newborn nurseries. "Because of this, more and more small babies are being reported as live births, and a large proportion does survive thanks to medical advances. However, because so many more small babies are being reported as live births, and a number do not survive past their first birthday, the rates of infant and neonatal mortality are not going down as much as they have in the past."
Throughout the 20th and into the 21st centuries, there has been a steep, continual decline in the number of babies who die before their first birthday. According to previous research published in Pediatrics, from 1915 to 2008, infant mortality rates in the United Sates decreased from 99.9 deaths to 6.6 deaths per 1,000 live births. The neonatal mortality rate decreased from 20.5 in 1950 to 4.3 deaths per 1,000 live births in 2008.
However, infant mortality and neonatal mortality rates in the United States plateaued from 2000 to 2008, from 6.6-7.0 and 4.3-4.7 deaths per 1,000 live births, respectively. Data from the Centers for Disease Control and Prevention shows that from 1983-2005, preterm birth rates increased from 9 percent to 12.7 percent, and the percentage of low birth weight infant births increased from 6.8 percent to 8.2 percent.
The research team sought to determine the impact of birth weight and the infant's gestational age at birth on mortality rates.
"We know that preterm birth and low birth weight are among the most frequent causes of infant and neonatal death in the United States," Carlo added. "What we did not know before this study was if the increasing proportion of very preterm births (babies born at less than 28 weeks' gestation) and very low birth weight (babies weighing less than 3.3 pounds at delivery), particularly those less than 1.1 pound, disproportionately affect mortality rates."
Using data compiled by the National Center for Health Statistics, Carlo and the research team looked at birth and infant death data for all the years available, 1983-2005. Years 1992-94 were unavailable in the database and excluded from the study. For the birth-weight specific neonatal and infant mortality rates, the data were analyzed following weight subgroups used in the database: 3,500 grams; 3,000-3,499 grams; 2,500-2,999 grams; 2,000-2,499 grams; 1,500-1,999 grams; 1,250-1,499 grams; 1,000-1,249 grams; 750-999 grams; 500-749 grams; and <500 grams. Infants with unknown birth weight were excluded.
For each year, the percentage of live births, percentage of infant deaths, percentage of neonatal deaths, infant mortality and neonatal mortality rates were calculated for each weight subgroup in the database. The same analysis was done for gestational age using the database subgroups: less than 28 weeks, 28-31 weeks, 32-35 weeks, 36 weeks, 37-39 weeks, 40 weeks, 41 weeks and 42 weeks.
The team then evaluated and compared low birth weight and prematurity trends over two time periods -- 1983-1999 and 2000-05 -- and their effect on infant and neonatal mortality. Analysis over these time periods, Carlo said, allowed the team to confirm whether or not the mortality rates had significantly changed over time, and to determine the contribution of low birth weight and preterm births to infant mortality and neonatal mortality rates.
The data, the team found, showed an increase in the number of infants in the lower birth weight subgroups and a corresponding decrease in infants that weighed more than 3,500 grams or 7.7 pounds. From 1983-2005, the number of very low birth weight infants -- those less than 1,500 grams or 3.3 pounds -- increased from 1.2 to 1.5 percent of all births and the proportion of live birth infants weighing less than 500 grams, or 1.1 pounds -- increased from 0.12 to 0.18 percent. However, the number of babies born weighing more than 3500 grams, or 7.7 pounds, decreased from 40.1 to 35.1 percent.
Additionally, Carlo said the team learned there was an increase in the number infants born weighing less than 1,500 grams, or 3.3 pounds, and the number of infant deaths from 1983-2005. Similarly, there was an increased in preterm infants born, and a corresponding decrease in term and post-term infants. The analysis confirmed that all of the subgroups of infants born weighing less than 1,500 grams, or 3.3 pounds, and infants born before 28 weeks gestation increasingly contributed to infant deaths from 1983-2005.
Carlo said it is important to point out that while the numbers of spontaneous preterm births of single fetus pregnancies have declined over the last century, there are tangible causes for the increase in premature and low birth weight infants reported in this paper.
"Preterm births in the United States have increased because women are being induced before 37 weeks gestation without a clear medically-necessary indication," Carlo said. "While a preterm birth might be considered an obstetric success, the benefits of this practice on perinatal mortality and morbidity need to be demonstrated. It is important to note that 23 percent of late-preterm infants do not have a recorded indication for early delivery noted in their birth certificate. Add to this the continuing increase in twin, triplets and higher order births, and you have the root causes of some of the increases in lower birth weight and preterm infants in the U.S. since the late 20th century."
Carlo added that in international comparison terms, this study showed the United States is not as behind in like countries as the numbers seem. The World Health Organization defines live birth as the expelled product of conception showing evidence of life regardless of the duration of pregnancy, which is also how live birth is defined in the U.S. Because other high-income countries define live birth as birth weight of 500 grams or more, the leveling off of mortality rates is more evident in the United States than in many other countries.
"The analysis clearly showed that the leveling off of infant and neonatal mortality is due to the increased proportion of extremely low birth weight and preterm infants born in the U.S.," Carlo said. "We were pleased to find that infant and neonatal mortality have continued to decrease, when birth weight and gestational age specific analysis is done, meaning normal birth weight and term infants are surviving past their first birthday in increasing numbers in the U.S."
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