Preterm Birth Causes, Consequences, and Prevention (2007) / Chapter Skim
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SECTION I Measurement : 2 Measurement of Fetal and Infant Maturity
SECTION I Measurement : 2 Measurement of Fetal and Infant Maturity
Pages 53-83
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From page 55... ...
Outcomes should be reported by gestational age categories, but birth weight for gestational age is an important indicator of the adequacy of fetal growth. Research on methods of quantifying fetal and infant maturity should be encouraged.
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From page 56... ...
, they are generally capable of sustained breathing, crying when hungry, sucking from a nipple, digesting milk, and complex physiological functions, including gas exchange, blood pressure control, glucose metabolism, and regulation of body fluids. Infants born preterm have immature organ systems that often need additional support to survive.
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From page 57... ...
In lieu of functional measures of fetal or infant maturity, accurate measures of gestational age are essential for clinical care as well as research on the causes, mechanisms, and outcomes of preterm birth. This chapter is devoted to clarifying definitions, describing methods of determining gestational age and their limitations, and demonstrating the implications of the use of precise definitions of the terms used.
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From page 58... ...
, United States, 1999 to 2000. Percentile based on gestational age calculated from LMP (first day of the mother's last menstrual period)
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From page 59... ...
. Most often, prenatal ultrasounds determine pregnancy duration with early measures of fetal size, when there is little individual variation in fetal growth.
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From page 60... ...
. LMP data are missing or incomplete on approximately 20 percent of certificates of live births in the United States, especially for women who are socioeconomically disadvantaged, who are most at risk for preterm birth and intrauterine growth restriction (IUGR)
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From page 61... ...
Chronological age is the age from the time of birth of an infant, whether the infant was born preterm or fullterm. Postmenstrual age, which suggests (but does not guarantee)
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From page 62... ...
Anthropometric Infant size measurements: birthweight, length, head circumference, foot length Postnatal Birth to 7 days External physical Infant exam Three to four days characteristics after birth to 40 weeks Postnatal PMA Neurological assessment: Infant exam Amiel-Tison Postnatal Birth to 4 or 5 days Combination: Dubowitz, Infant exam Ballard, New Ballard Score and others*
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From page 63... ...
. Early prenatal ultrasounds before 20 weeks gestation are more accurate (95 percent confidence interval = ± 3 to 5 days)
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From page 64... ...
Early prenatal ultrasounds require early prenatal care. In the United States, only 84 percent of pregnant women receive prenatal care during the first trimester, and 3.5 percent did not access prenatal care until the third trimester or had no prenatal care (CDC, 2004d)
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From page 65... ...
from instituting routine prenatal ultrasounds before the third trimester (there appear to be no benefits of routine late prenatal ultrasounds) can be considered against the benefits of better assessments of gestational age, earlier detection of multiple pregnancies and detection of unsuspected fetal malformations before the third trimester (Neilson, 1998; Bricker and Neilson, 2000)
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From page 66... ...
. Other amniotic fluid tests for fetal lung maturity include the shake test; lamellar body count; and measurement of the phosphatidylglycerol, saturated phosphatidylcholine, fluorescent polarization, or lung phospholipid profile (Torday and Rehan, 2003; Wijnberger et al., 2001)
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From page 67... ...
and the Dubowitz gestational age assessment (Dubowitz et al., 1970) , overestimate gestational age by 2 or more weeks in 45 to 75 percent of preterm infants with birth weights less than 1,500 grams (Sanders et al., 1991; Shukla et al., 1987; Spinnato et al., 1984)
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From page 68... ...
. The difficulty of using postnatal measures of degree of maturation of external physical characteristics and neurological muscle tone to estimate gestational age at birth highlights the difference between pregnancy duration and degree of maturation (Allen, 2005a)
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From page 69... ...
HETEROGENEITY OF THE PRETERM INFANT POPULATION Intrauterine Growth Restriction, Small for Gestational Age, and Fetal Maturation Intrauterine growth restriction (IUGR, also known as fetal growth restriction) is as complex and multifactorial a condition as preterm delivery, and many of its etiologies and mechanisms are just as poorly understood.
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From page 70... ...
A change from the use of data generated from infants born at a given gestational age less than 36 weeks to the use of weight data estimated from prenatal ultrasounds at that gestational age increases the proportion of infants diagnosed at birth as being small for gestational age from 10 to 25 percent. This approach has not been widely adopted, but these data raise a convincing argument for the use of the ultrasound estimates of fetal weight data to define small for gestational age (Bernstein, 2003)
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From page 71... ...
. Infants born with IUGR before 34 weeks gestation have greater mortality and morbidity than preterm appropriate for gestational age infants of the same gestational age (Garite et al., 2004; Tyson et al., 1995)
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From page 72... ...
. Infants born at 22 to 25 weeks gestation die if they are not resuscitated at birth and provided with neonatal intensive care.
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From page 73... ...
However, many infants born at 22 to 25 weeks gestation die shortly after delivery, and are never admitted to an NICU. In addition, a large proportion of infants born at a gestational age of less than 23 weeks or with a birth weight of less than 500 grams are stillborn (60 to 89 percent and 68 to 77 percent, respectively)
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From page 74... ...
. Marked regional variations in the management and the rates of survival of infants born at the lower limit of viability and variations in the methods used to estimate gestational age make it difficult to evaluate trends over time with respect to live birth rates by gestational age and fetal death rates (Costeloe et al., 2000; Lorenz et al., 2001; Sanders et al., 1998; Tyson et al., 1996)
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From page 75... ...
births were infants born at 33 to 36 weeks gestation, whereas only 3 percent were born at gestational ages of less than 33 weeks. As many as 34 percent of twins are born at 32 to 35 weeks gestation, 31 percent are born at 36 to 37 weeks of gestation, and only 24 percent are born after 37 weeks gestation (Min et al., 2000)
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From page 76... ...
Recognizing the higher mortality and morbidity rates for late preterm infants than fullterm infants, health care providers and families need to weigh carefully the advantages of earlier delivery against the health, financial, and economic costs of preterm delivery. IMPLICATIONS FOR PUBLIC HEALTH AND RESEARCH Although the complex interplay between the duration of pregnancy, fetal and infant size and maturity, and how they are measured are sources of some confusion, evaluation of the interrelationships among these factors provides an opportunity to gain some insight into the factors contributing to preterm birth.
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From page 77... ...
Some have suggested that many of the obstetric and neonatal intensive care advances (e.g., surfactant and antenatal steroid use) disproportionately improved the rates of survival for white preterm infants (who had higher incidences of RDS within each gestational age category)
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From page 78... ...
role. Postnatal estimates of gestational age, especially the Ballard and Dubowitz gestational age assessments, tend to overestimate gestational age in preterm infants, but the magnitude of the overestimation varies with race and ethnicity.
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From page 79... ...
Although birth certificates are intended to establish the date of birth, the citizenship, and the nationality of a newborn infant, they contain valuable public health information and are the only national source of birth weight and gestational age data. Large state and national population databases with birth and death certificate data have been used to plot gestational age distributions, birth weights for gestational age, and gestational age- and birth weight-specific neonatal mortality rates.
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From page 80... ...
Agencies and researchers will need to consider these changes when they analyze the trends and state-to-state comparisons using gestational age. Fortunately, the change in gestational age determination from the use of LMP to ultrasound-based data has less of an effect on the preterm birth rate than it does on the birth rate for postterm infants born after 41 weeks gestation.
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From page 81... ...
Missing data regarding initiation of prenatal care vary from 17 percent of the records of fetal deaths at 20 to 27 weeks of gestation to 11 percent of fetal deaths beyond 27 weeks of gestation and 2.8 percent of live births. The possibility exists that changing practices in the categorization and reporting of live births and fetal deaths have contributed to falling fetal death rates and rising preterm birth and infant mortality rates in the United
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From page 82... ...
Similarly, intensive prenatal care of high-risk mothers facilitates the detection of fetuses whose adaptive systems become overwhelmed by adverse intrauterine circumstances. An indicated preterm delivery that prevents a fetal death is not an indicator of worsening infant health, even if it does contribute to a higher preterm birth rate Consideration should be given to using perinatal mortality rates as another child health indicator.
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From page 83... ...
Every effort should be made to improve the quality of national vital records, especially data on the gestational ages of newborns and the rates of preterm births. Uniform data collection and reporting procedures facilitate comparisons among states, over time, and with data from other countries.
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