Premature infants are those who are born too soon, that is, born before the normal length of time in pregnancy that is typically needed for a fetus to develop, mature, and thrive postnatally. The average length of pregnancy is thirty-nine to forty weeks, which is the approximate duration of pregnancy needed for the fetus to reach full development and maturity. Infants delivered between the thirty-seventh and forty-first week of pregnancy are typically referred to as ''term'' or ''mature'' births. ''Preterm'' or ''premature'' birth is defined as delivery before the thirty-seventh week of pregnancy and ''postterm'' or ''postmature'' births are those occurring at forty-two weeks and beyond. Preterm deliveries are further delineated as either ''very preterm'' (before the thirty-third week) or ''moderately preterm'' (between the thirty-third and thirty-sixth weeks).
The use of the expression ''prematurity'' has changed over time. It once was used to refer to any early or small birth, thereby encompassing births that occurred before term and births of infants of low birthweight (less than 2,500 grams [5 pounds, 8 ounces]). It became apparent, however, that not all low birthweight infants were born preterm and that not all preterm births were low birthweight. Many low birthweight infants are term births that are small in size due to growth-related complications. These two different types of low birthweight infants—those preterm and those small for their duration of gestation— were recognized to reflect distinct medical problems, and as a result, more specific labels were needed to refer to each type of birth according to its birth-
weight, duration of gestation, and birthweight for gestation. It is now the convention to clearly distinguish between births that are preterm, low birth-weight, and/or ''small for gestational age.'' For single live births born to U.S. resident mothers between 1995 and 1997, 9.6 percent were preterm and 6.1 percent were low birthweight. Of these low birth-weight deliveries, 61.9 percent were preterm. Simultaneously, only 39 percent of preterm births were low birthweight.
The duration of the pregnancy prior to delivery in completed weeks is referred to as the gestational age of the newborn and thereby establishes if the delivery is preterm. The duration of pregnancy is traditionally measured as the interval from the date of the mother's last menstrual period to the date of birth. This approach for defining gestational age at delivery is derived from obstetrical practice and overestimates by approximately two weeks the interval from conception to birth. Alternate approaches to estimating the gestational age have been developed and include physical and neurological assessments of the newborn and prenatal ultrasound measures of fetal size (body length, femur length, and skull diameter and circumference).
Knowledge of gestational age is essential for the appropriate medical management of both the pregnancy and the newborn infant as gestational age serves as a proxy measure for the extent of fetal development and the fetus's readiness for birth. As an indicator of newborn maturity, gestational age is closely associated with the newborn's chances for survival during the first year and the likelihood of developing neonatal complications. Moreover, knowledge of a preterm infant's gestational age is necessary for assessing developmental progress in infancy.
Gestational age is also used by public health professionals to calculate a variety of statistical indicators that are useful for monitoring the health status of populations and assessing the need for and impact of targeted public health interventions. Preterm and very preterm percentages in populations may reflect the prevalence of a variety of health-related concerns, including infections, psychosocial and physical stresses, poor nutrition, and substance abuse.
Between 1981 and 1996 in the United States, annual preterm rates rose from 9.4 percent to 11 percent of live births, a 17 percent increase (see Figure 1). Although infant mortality rates declined during the same period, the ongoing increase in preterm rates is a matter of considerable concern. Indeed, increasing rates of preterm birth are recognized as a pregnancy related crisis in the United States. Approximately three-quarters of neonatal deaths and nearly one-half of the long-term neurological damage seen
in children have been attributed to preterm birth. Preterm infants have an increased risk of developmental delay, cerebral palsy, mental retardation, sensory impairment, learning and school-related problems, and other disabilities. Moreover, the health-care costs associated with an extremely small baby or early birth are more than ten times greater than those of normal weight infants. The persistent disparity in infant mortality rates among ethnic groups in the United States has also been related to ethnic differences in preterm birthrates. Further, high preterm birthrates in the United States have been identified as a major contributor to this nation's relatively poor ranking for infant mortality among developed countries.
Considerable effort was expended in the United States during the last few decades of the twentieth century to reduce the rates of preterm delivery and low birthweight. In 1985 the Institute of Medicine promoted increasing access to prenatal care and improving the content of care as a means to improve the rates of these adverse perinatal outcomes. Since then, considerable effort has been expended to improve prenatal care and reduce preterm births. Between 1981 and 1995, the percent of women starting prenatal care in the first trimester increased 6.1 percent, from 76.9 percent to 81.6 percent. Over the same period, the percent of women with adequate utilization of prenatal care increased 37.9 percent, from 29.3 percent to 40.4 percent, while the percent of women who inadequately used prenatal care decreased 33.3
percent, from 13.5 percent to 9 percent. Notwithstanding improvements in access to and use of prenatal care, preterm and low birthweight rates in the United States continued to increase (see Figure 1). By 1995, a Future of Children report on low birthweight concluded that prenatal care in its present form ''does little to prevent low birth weight or preterm birth'' (Shiono and Behrman 1995). Other reviews in the late 1990s also concluded that there is little done during the standard prenatal care visit to reduce the risk of very low birthweight or to prevent preterm delivery.
Several explanations can be proposed for the ongoing increase in preterm birthrates in the United States. The possible factors driving these trends include:
• changes in vital record reporting (i.e., very pre-term infants once reported as fetal deaths are now being registered as live births);
• a rising incidence of multiple births, the infants from which are more likely to be preterm;
• an increase in the percentages of unmarried and older aged mothers, who are at greater risk of having a preterm delivery; and
• changes in obstetric practice that have lead to earlier delivery of pregnancies deemed at risk of a poor outcome.
Improvements in the early and adequate use of prenatal care services, coupled with developments in obstetric and neonatal practice and technology (e.g., surfactant, steroids, ultrasound), may lead to earlier diagnosis of problems and resultantly to an earlier ce-sarean delivery, which, while further decreasing the risk of maternal, fetal, and even infant mortality, may potentially increase the rate of moderate preterm delivery. In all, the rise in preterm birthrates in the United States during the 1990s may stem largely from attempts to improve reporting, fertility, and survival rather than from a major rise in high-risk behavioral and medical factors.
The limited success of the efforts to reduce rates of preterm birth in part stems from preterm birth being a single outcome (i.e., being born too early) that results from multiple causes, most of which are still poorly understood. Only between 25 and 40 percent of preterm births can be explained with currently known risk factors, including single marital status; low socioeconomic status; previous preterm birth; maternal illness (e.g., hypertension); cocaine and tobacco use; multiple second trimester spontaneous abortions; gestational bleeding; urogenital infections; multiple gestations; placental, cervical, and uterine abnormalities; and black race of mother (which may reflect a complex array of socioeconomic, cultural, biological, and behavioral risk characteristics). The major clinical classifications of preterm birth are spontaneous preterm labor, preterm rupture of membranes before the onset of labor, and indicated preterm delivery for pregnancy complications. Nevertheless, as each of these clinical presentations have multiple causes, more recent efforts have focused on establishing the many unique antecedents and biological causes for preterm birth. Accordingly, the discovery and development of a single prevention or intervention strategy to markedly reduce the present level of preterm births is highly unlikely and probably unrealistic. In spite of the many risk factors for pre-term birth that have been identified, only a few of these risk factors, such as cigarette smoking, can be considered modifiable during the current pregnancy. In the early twenty-first century, research efforts to further understand the determinants of preterm birth are focusing on the role of infections, stress, socioeconomic deprivation, pregnancy anxiety, hormones, nutrition, and fetal growth restriction.
See also: BIRTHWEIGHT; HIGH RISK INFANTS; INFANT MORTALITY; PREGNANCY
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Greg R. Alexander Mary Ann Pass Martha Slay
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