eMedicine Specialties > Pediatrics: Cardiac Disease and Critical Care Medicine > Neonatology
Prematurity
Updated: Jun 19, 2009
Introduction
Background
Prematurity refers to the broad category of neonates born at less than 37 weeks' gestation. Although the estimated date of confinement (EDC) is 40 weeks' gestation, the World Health Organization (WHO) broadened the range of full term to include 37-42 weeks' gestation.
Premature newborns have many physiologic challenges when adapting to the extrauterine environment. Most articles in the neonatology section discuss in detail the most serious of these problems. Serious morbidities occur in extremely low birth weight (ELBW) infants. For more information, see Extremely Low Birth Weight Infant, Acute Respiratory Distress Syndrome, Bronchopulmonary Dysplasia, Periventricular Hemorrhage-Intraventricular Hemorrhage. The near-term neonate (34-36 weeks' gestation) has issues of prematurity that include feeding immaturity, temperature instability, and prolonged jaundice. This article provides a general overview of the premature infant.
Pathophysiology
Before birth, the placenta serves 3 major roles for the fetus: provision of all the nutrients for growth, elimination of fetal waste products, and synthesis of hormones that promote fetal growth.
With the exception of most electrolytes, the maternal circulation contains more substrate (eg, blood glucose) than the fetal circulation. In addition, the placenta is metabolically active and consumes glucose. Waste products of fetal metabolism (eg, heat, urea, bilirubin, carbon dioxide) are transferred across the placenta and eliminated by the mother's excretory organs (ie, liver, lung, kidneys, skin).
In addition, the placenta acts as a barrier to infection through mucosal macrophages and by allowing transfer of maternal immunoglobulins (immunoglobulins such as immunoglobulin G [IgG]) to the fetus beginning at 32-34 weeks' gestation. Placental dysfunction is involved in the transfer of IgG. Antibacterial activity of the amniotic fluid improves as gestational age advances.
Each of the immature organs of a premature infant has functional limitations. The tasks of caregivers in neonatal intensive care units (NICUs) are to recognize and monitor the needs of each infant and to provide appropriate support until functional maturity can be achieved.
Frequency
United States
In the general population, 12% of infants are born prematurely.
International
No reliable numbers are available because different countries use different definitions of birth (eg, survival after birth, survival after 1 month).
Mortality/Morbidity
The mortality rate is high in developing countries, especially those of Sub-Saharan Africa. The perinatal mortality rate is 70 deaths per 1000 births; the neonatal mortality rate is 45 deaths per 1000 live births. Preterm birth is the strongest independent predictor of mortality in the United States. Preterm delivery accounts for 75-80% of all neonatal morbidity and mortality.
Since the early 1960s, survival rates of premature infants substantially increased because of technologic advances. From 1989-1990, infants with birth weights less than 751 g had a survival rate of 39% (range among centers, 23-48%). In 1992, the US Food and Drug Administration (FDA) approved exogenous surfactant therapy for respiratory distress syndrome (RDS), leading to a considerable improvement in survival rates. Since the FDA approved the use of surfactant and since the subsequent introduction of numerous natural surfactants, the mortality rate attributed to surfactant deficiency has been markedly reduced. See Acute Respiratory Distress Syndrome.
Data from the Vermont Oxford Network in 1994-1996 indicated that the survival rate of infants born weighing less than 1000 g was 74.9%.1 Survival of infants born weighing less than 1000 g and requiring cardiopulmonary resuscitation in the delivery room was substantially decreased (53.8%). The changes in obstetric and neonatal care in the first half of the decade of 1990s decreased mortality and morbidity for ELBW infants. No additional improvements in mortality and morbidity were observed at the end of the decade.
Obstetric and pediatric personnel must be familiar with their own institutional data in addition to national benchmarks related to gestational age and mortality rates. These data are essential for proper prenatal counseling of parents and/or caregivers regarding survival and resuscitation plans.
The 3 primary causes of mortality in infants born with a weight of less than 1000 g are respiratory failure, infection, and congenital malformation. Infection of the amniotic fluid leading to pneumonia is the major cause of mortality.2 In infants who weigh less than 500 g at birth, immaturity is listed as the only cause of mortality. See Ethical Issues in Neonatal Care.
Women who have an intrauterine infection do not respond to tocolytics. Preterm premature rupture of membranes (PPROM) is associated with 30-40% of premature deliveries. See Premature Rupture of Membranes. Mortality of the premature infant increases with coexisting PPROM but depends on gestational age and the expertise of the maternal-fetal monitoring team. Postnatal findings of periventricular leukomalacia (PVL) on cranial ultrasonography are highly correlated with chorioamnionitis.
In premature infants with a congenital heart defect (CHD), excluding isolated patent ductus arteriosus, the actuarial survival rate is 51% at 10 years, whereas infants with both CHD and prematurity have substantially worsened outcomes than infants who only have one of these conditions.3 The survival rate improved as the study period (1976-1999) progressed. Congenital anomalies are an independent risk factor for mortality and morbidity in preterm birth.
In a longitudinal study of 1279 extremely premature children, (gestational age ≤28 wk; birth weight <1250 g), Robertson et al found permanent hearing loss in 3.1% and severe-to-profound loss in 1.9%.4 Among affected children, hearing loss was delayed in onset in 10% and progressive in 28%. Prolonged supplemental oxygen use was the most important marker for predicting hearing loss.Race
Premature infants are born to women of every race. ELBW infants are most commonly born to women of low socioeconomic status, black women, teenaged female adolescents, and mothers older than 40 years. Women at highest risk of premature delivery can be assessed by using a scoring system that reviews their socioeconomic status, history, daily habits, and current pregnancy events.3 About 30% of women with a high-risk score deliver prematurely compared with 2.5% of women with a low-risk score.
Primarily because of the increased incidence of preterm infants, the overall neonatal mortality rate in blacks in the United States is 2.3 times that of whites. Improvements in socioeconomic status and perinatal care have not improved the rate of prematurity and infant mortality rate in this population.
Sex
Female sex is associated with increased rates of survival of newborns born at 22-25 weeks' gestation.
Clinical
History
- In assessing prematurity, gestational age dating by using the mother's history can be unreliable because of uncertainty of the dates.
- About 20% of women have an uncertain last menstrual period (LMP).
- Gestational age assessment begins prenatally with obstetric ultrasonography in the first trimester.
- Discovery of many fetal anomalies, unsuspected multiple gestation, location of the placenta, and an accurate dating of the pregnancy are additional major benefits of early ultrasonography.
Physical
Confirmation of gestational age is based on physical and neurologic characteristics. In 1979, the Dubowitz scoring system for determining gestational age based on neurologic and physical parameters was revised to include 12 items.5 The Ballard Scoring System, revised again to include extremely low birth weight (ELBW) infants, remains the main tool clinicians use after delivery to confirm gestational age by means of physical examination.2
- The major parts of the anatomy for physical characteristic markers are ear cartilage (see Media files 1-3), sole creases (see Media files 4-6), breast tissue (see Media files 7-8), and genitalia.
Preterm infant at 28 weeks' gestation. Note the small amount of ear cartilage and/or flattened pinna.
Preterm infant at 33 weeks' gestation. Note the increased cartilage, recoil, and outer ridge curving inward.
A term infant has well-developed cartilage with instant recoil.
Preterm infant at 28 weeks' gestation. Note the flat sole.
Preterm infant at 33 weeks' gestation. Note the presence of only an anterior crease.
Term gestation. Note the multiple creases.
Preterm infant at 28 weeks' gestation. No breast tissue is present, and the areolae are barely visible.
Preterm infant at 33 weeks' gestation. The breast tissue is less than 1 cm, and the areolae are raised and/or pigmented.
- This examination should be performed immediately after stabilization and before the expected weight loss occurs on the first day.
- Hittner et al reported that regression of the vascularity of the lens capsule is an excellent tool to confirm a gestational age of 28-34 weeks.6
- Neurologic criteria include muscle tone of the trunk and extremities and joint mobility.
- Reassessing the neurologic criteria 18-24 hours after birth is best to allow for recovery from maternal medication (eg, magnesium sulfate, analgesics), which may decrease tone and responsiveness.
Causes
Premature delivery can be the result of preterm labor and preterm premature rupture of the membranes (PPROM) or can be due to maternal indications (eg, pregnancy-induced hypertension).
- Chorioamnionitis
- Amniocentesis that demonstrates bacteria, WBCs, and a low glucose concentration confirms the diagnosis of chorioamnionitis and is an indication for delivery.
- A decrease in the biophysical score or profile in association with chorioamnionitis is associated with fetal infection.
- Rates of perinatal mortality, neonatal infection, and respiratory distress syndrome (RDS) increase in the presence of maternal fever and chorioamnionitis.
- Intrauterine growth restriction (10th percentile for birth weight): This is significantly associated with perinatal mortality and long-term morbidity.
- Low socioeconomic status: Programs offering additional social support for at-risk pregnant women have not been demonstrated to reduce the numbers of ELBW or preterm infants.
- Maternal diabetes
- Pregnancies complicated by diabetes and poor glycemic control are associated with a high incidence of prematurity, macrosomia, malformation, fetal death, and neonatal death.
- The rate of preterm birth (<37 weeks' gestation) is 20-22% of persons with insulin-dependent diabetes.
- In women with diabetes diagnosed before pregnancy, the frequency of preeclampsia is increased as the severity of diabetes increases.
- Multiple gestation pregnancies
- Women with multiple gestation pregnancies are at high risk of preterm labor and delivery and account for increasing percentage of preterm births and ELBW infants.
- With advances in assisted reproductive technology, multiple gestation pregnancies have increased.
- Preterm birth rate for twins has increased from 40.9% in 1981 to 55% in 1997. Multiple births related to infertility treatment have dramatically increased.7
- Prepregnancy counseling of prospective parents regarding the risks related to multiple gestations is important.
- Preterm birth (<35 weeks' gestation) occurs in 26% of twins compared with 3% of singletons.
- Triplet pregnancies are associated with an increased incidence of preterm labor and delivery at a decreased gestational age and birth weight, compared with singletons and twins. When the data are controlled for gestational age, outcomes are similar for singletons, twins, and triplets.
- Maternal age
- In women aged 13-15 years, the rate of preterm birth is 5.9%. This rate declines to 1.7% in women aged 18-19 years and 1.1% in women aged 20-24 years.
- The rate of preterm births increases in pregnancies in which the mother is older than 40 years. The scoring system for the risk of preterm delivery uses a criterion of age older than 40 years.
- Tobacco use
- Approximately 15-20% of pregnant women smoke tobacco.
- Tobacco use is a risk factor for placental abruption and accounts as a factor for 15% of preterm births and 20-30% of ELBW infants.
More on Prematurity |
 Overview: Prematurity |
| Differential Diagnoses & Workup: Prematurity |
| Treatment & Medication: Prematurity |
| Follow-up: Prematurity |
| Multimedia: Prematurity |
| References |
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Further Reading
Keywords
prematurity, preterm, immature, low birth weight, LBW, extremely low birth weight, ELBW, pneumonia, respiratory distress syndrome, acute respiratory distress syndrome, ARDS, bronchopulmonary dysplasia, periventricular hemorrhage, intraventricular hemorrhage, jaundice, pneumonia, respiratory failure, premature rupture of membranes, PPROM, periventricular leukomalacia, PVL, congenital heart disease, patent ductus arteriosus, hearing loss, multiple gestation, multiple births, maternal diabetes, maternal chorioamnionitis, intrauterine growth retardation, treatment, diagnosis
