A premature infant is defined as one born before 37 weeks of gestation (pregnancy ) without regard to birth weight. The length of a full-term pregnancy ranges from 37 to 42 weeks, measuring from the first day of the last menstrual period. "Preterm" is a word that is sometimes used instead of "premature." Extremely premature infants are defined as those born between 22 and 28 weeks of gestation. As of 2001, babies born at 21 weeks of gestation or less have little chance of survival.
In the United States, about 10% of all infants are born prematurely. African American babies are more likely to be premature (14%) than Caucasian or Hispanic babies (7%). The rates of survival of premature infants, however, have increased dramatically over the last three decades. At one teaching hospital in the Midwest, the survival rate of infants weighing less than 1 lb 12 oz (800 grams) at birth rose from 20% in 1977 to 49% in 1990. In spite of advances in medical technology, these children remain at higher risk of birth defects, weakened immune systems, and a variety of chronic medical and developmental disorders. Many require long-term follow-up care.
Chances for survival
The most important factor affecting survival in extremely premature infants is gestational age at the time of birth, which is defined as the estimated time elapsed since conception. Another term for gestational age is postconceptional age. The likelihood of a preterm infant's survival at specific gestational ages in the United States in the late 1990s is as follows:
- 21 weeks or less: 0% survival rate
- 22 weeks: 0-10% survival rate
- 23 weeks: 10-35% survival rate
- 24 weeks: 40-70% survival rate
- 25 weeks: 50-80% survival rate
- 26 weeks: 80-90% survival rate
- 27 weeks: greater than 90% survival rate
The baby's chances of survival increase 3-4% per day between 23 and 24 weeks of gestation, and 2-3% per day between 24 and 26 weeks.
In addition to gestational age, the baby's weight at birth, the presence of breathing problems, the presence of birth defects, and the presence of severe infection are important factors influencing survival. Birth weight in premature infants is categorized as follows: birth weight below 5 lb 8 oz (2500 g) is defined as low birth weight (LBW); weight below 3 lb 5 oz (1500 g) is very low birth weight (VLBW); and weight below 2 lb 3 oz (1000 g) is extremely low birth weight (ELBW).
Other factors affecting survival
Other factors that influence the rate of organ development or the fetal oxygen supply also influence a premature infant's chances for survival. These factors include:
- rupture of the amniotic sac with loss of amniotic fluid before 24 weeks of gestation
- male sex (male infants are slower to mature)
- race (African American infants have slightly better chances of survival than Caucasian infants of the same birth weight)
- uncontrolled diabetes in the mother (slows organ development in the fetus)
- severe hypertension before the eighth month of pregnancy (slows delivery of nutrients and oxygen to the fetus)
Causes and symptoms
Causes of preterm labor
Labor that begins before the 37th week of pregnancy is called premature or preterm labor. It is responsible for about 85% of illnesses and deaths in newborns in the United States. Premature labor is sometimes induced because of the mother's or the infant's condition. Preeclampsia/eclampsia is the most common reason for inducing labor; other reasons include fetal distress or bleeding. Common causes of spontaneous premature labor include:
- abruptio placentae, or detachment of the placenta from the uterine wall
- premature rupture of the amniotic sac
- incompetent (too easily dilated) cervix
- multiple pregnancy
- abdominal or cervical surgery during the current pregnancy
- placenta previa (the placenta lies between the baby and the birth canal)
Factors that increase the mother's risk of preterm labor include:
- history of preterm delivery
- listory of abortions or miscarriages
- heavy smoking (more than 10 cigarettes per day)
- history of drug abuse
- exposure to diethylstilbestrol (DES), a synthetic estrogen given to treat estrogen deficiency conditions
- urinary tract infection
- height below 5 ft (1.5 m)
- weight below 100 lb (45 kg)
- age below 18
Common medical problems in premature infants
The most common problems in premature infants include jaundice, apnea (a pause in breathing lasting longer than 15-20 seconds), and inability to breast-feed or bottle-feed. Apnea in premature infants is accompanied by the baby's turning pale or bluish, and by a slowing-down of its heart rate (bradycardia). These problems are particularly likely to affect infants born more than four to six weeks early.
More serious medical problems that are relatively frequent in premature infants are described in the next four subsections.
RESPIRATORY DISTRESS SYNDROME (RDS). Respiratory distress syndrome (RDS) is the most common lung disorder in preterm infants. It is caused by a lack of surfactant in the lungs. Surfactant is a surface-active substance produced by the body that coats the lungs and keeps them from collapsing. Babies with RDS typically breathe rapidly, with flaring nostrils and grunting sounds. RDS is usually treated by giving the baby extra oxygen under pressure. Sometimes the baby is also given additional surfactant by intubation.
TRANSIENT TACHYPNEA OF THE NEWBORN (TTNB). Transient tachypnea of the newborn is a disorder lasting for several hours or a few days characterized by rapid, grunting breathing. TTNB is thought to be caused by slow reabsorption of fetal lung fluid. It is also treated with supplemental oxygen.
PATENT DUCTUS ARTERIOSUS (PDA). A patent ductus arteriosus refers to an opening in the blood vessel that connects the aorta and the pulmonary artery. In full-term infants, this blood vessel closes in the first few days after birth. In preterm infants, it may remain open, thus allowing too much blood to flow into the baby's lungs. PDAs are treated with indomethacin or ibuprofen to close the blood vessel, or diuretics to decrease the amount of fluid that collects in the baby's lungs. If the medications do not close the ductus, it can be closed surgically.
RETINOPATHY OF PREMATURITY (ROP). Retinopathy of prematurity, or ROP, is the abnormal growth of blood vessels in the eyes. ROP is most common in infants who are more than 12 weeks premature. It often resolves on its own, but sometimes requires treatment. Treatment consists of killing the inner lining of the eye at the ends of the abnormal blood vessels to prevent further growth.
Less common but severe medical conditions
The next five subsections briefly describe serious disorders that may affect premature infants.
AIR LEAKS. Air leaks refer to several conditions in preterm infants, all characterized by air leaking from the air sacs in the lungs. The air may be trapped between the chest wall and the lung, trapped in the middle part of the chest, leaked into the abdomen, or leaked into the spaces between the tiny air sacs. Premature infants are vulnerable to air leaks because their lungs are not yet fully developed. In milder cases, the air is gradually reabsorbed by the baby's body. In severe cases, the baby may be treated by placement of a chest tube, or be placed on a ventilator.
NECROTIZING ENTEROCOLITIS (NEC). Necrotizing enterocolitis (NEC) is an inflammatory disorder in which part of the bowel lining or part of the bowel itself is destroyed. It is not always clear why a specific infant may develop NEC. The baby may vomit, have a swollen or abnormally reddish abdomen, or pass blood in the stool. NEC is usually treated with antibiotics. If a section of the bowel itself has been destroyed, surgery may be necessary.
BRONCHOPULMONARY DYSPLASIA (BPD). Bronchopulmonary dysplasia, or BPD, is a long-term lung disease that is most likely to develop in infants who were extremely premature, had severe RDS, or developed infections of the lungs. BPD is diagnosed if the baby's chest x-rays remain abnormal and the baby still needs oxygen by the time it is 36 weeks of gestational age (a month before its full-term due date). Babies with BPD are treated with supplemental oxygen, sometimes for as long as a year after they develop BPD. They may also be given steroids or diuretics.
INTRAVENTRICULAR HEMORRHAGE (IVH). Intraventricular hemorrhage (IVH) is a brain disorder in which blood seeps into the ventricles (a series of connecting cavities) of the brain. IVH develops because the blood vessels in the brain of a premature infant are fragile and break open easily. The preterm babies at highest risk for IVH are those weighing less than 2 lb 4 oz (1000 g). There is no specific treatment for IVH, but the condition can be monitored by ultrasound. In mild cases of IVH, the blood in the ventricles is slowly reabsorbed by the body. Babies with hydrocephalus (abnormal amounts of cerebrospinal fluid collecting in the ventricles of the brain), which is a possible complication of IVH, are at risk for permanent brain injury.
PERIVENTRICULAR LEUKOMALACIA (PVL). Peri-ventricular leukomalacia (PVL) refers to a softening of the white matter of the brain surrounding the ventricles, caused by the death of brain tissue in these areas. The precise causes of PVL are still not fully understood. PVL often develops in babies with IVH. There is no specific treatment for PVL; moreover, infants with this disorder are at very high risk for motor (movement) and developmental disabilities as they mature.
Many of the problems associated with prematurity depend on the degree of prematurity and the baby's birth weight. The gestational age of the fetus may be calculated from the date of the mother's last menstrual period or by using ultrasound imaging to observe fetal development. After the baby is born, such physical assessment scales as the Dubowitz Maturity Scale may be used to estimate gestational age. The Dubowitz scale bases its determination on the infant's physical and neuromuscular maturity.
Once the baby's gestational age and weight are determined, further tests and electronic monitoring may need to be used to diagnose problems or to track the baby's condition. A heart monitor or cardiorespiratory monitor may be attached to the baby's chest, abdomen, arms, or legs with adhesive patches to monitor breathing and heart rate. A thermometer probe may be taped on the skin to monitor body temperature. Blood samples may be taken from a vein or artery. A radiologic technologist may perform x rays or ultrasound imaging to examine the heart, lungs, and other internal organs.
Treatment depends on the types of complications that are present. The infant may be placed in a heat-controlled unit (an incubator) to maintain body temperature. Infants that are having trouble breathing on their own may need oxygen either pumped into the incubator, administered through small tubes placed in their nostrils (nasal cannula), or through a respirator or ventilator that pumps air through an endotracheal tube inserted into the airway. Oxygen may be delivered under pressure by continuous positive airway pressure (CPAP) or positive end expiratory pressure (PEEP).
Medications and surgery
The infant may require fluids and nutrients to be administered through an intravenous line inserted into a vein in the hand, foot, arm, leg, or scalp. If the baby needs medications to treat infections, to close a patent ductus, or to increase urinary output, they may also be administered through the intravenous line. Surgery may be required in the treatment of PDA, NEC, or IVH. If hydrocephalus develops as a complication of IVH, it may be treated by surgical placement of a shunt, which is a tube connecting one of the ventricles in the brain to a longer tube under the skin that allows the excess cerebrospinal fluid to be absorbed in the abdomen.
Premature infants require special attention to their physical and social environment as well as to the symptoms of any disorders they may have. Some modifications are necessary because the nervous systems of preterm infants are not as fully developed as those of full-term infants.
PHYSICAL ENVIRONMENT. Premature infants experience loud noises and bright lights as stressful. In addition, they are more disturbed by frequent handling than full-term infants. Parents and other care givers should be advised to position the infant on its side in a flexed position; because premature infants do not have the muscle strength to move against gravity, they tend to lie with arms and legs in an extended position. Over a period of time, this extended position can delay the baby's motor development.
SOCIAL ENVIRONMENT. Neonatal intensive care units, or NICUs, complicate the premature infant's social environment by exposing him or her to many more clinical staff members than the full-term infant, and at the same time keeping him or her away from the parents more than the full-term infant. For this reason, skin-to-skin contact with parents, sometimes called kangaroo care, and gentle massage are encouraged as promoting infant and parent well-being. Parents should be encouraged to have early contact with the premature baby to facilitate parent-infant bonding.
The prognoses of premature infants vary widely, depending on gestational age, birth weight, the reasons for premature delivery, and the many other factors discussed above.
Health care team roles
Premature infants receive routine care and monitoring in the NICU from nurses. A neonatologist, who is a physician specializing in care of the newborn, may be consulted if a medical intervention seems necessary. A radiologist may be consulted for radiographic or ultrasound studies, and a surgeon may be called in if an operation is required.
Other health professionals involved in the premature infant's care are the respiratory therapist, who monitors the care of infants requiring supplemental oxygen or ventilators; the social worker, who helps families adjust emotionally and provides referrals to hospital and community resources; and the occupational therapist, who evaluates the baby's progress and plans a program of developmental therapy if necessary.
Apnea— A pause in breathing of more than 15-20 seconds. In premature infants, apnea usually causes a change in the baby's color and a slowing of the heartbeat.
Bronchopulmonary dysplasia (BPD)— A chronic lung disorder that sometimes develops in premature infants who have had severe respiratory distress syndrome or lung infections.
Chronologic or birth age— The infant's age as measured by the time elapsed since birth.
Gestational age— The infant's age as measured by the estimated time since conception; sometimes called postconceptional age.
Hydrocephalus— An abnormal buildup of cerebrospinal fluid in the ventricles of the brain. In premature infants, it is often a complication of IVH.
Intraventricular hemorrhage (IVH)— A condition in which fragile blood vessels within the brain burst and bleed into the hollow chambers (ventricles) of the brain and into the tissue surrounding them.
Kangaroo care— A form of skin-to-skin contact in which either parent of a premature infant holds the baby under the blouse or shirt, against the skin. It is thought to help the infant's development as well as its bonding with the parents.
Necrotizing enterocolitis (NEC)— A condition that sometimes occurs in premature infants in which the lining of the bowel or a section of the bowel itself dies.
Neonatology— The study of the development and disorders of newborn children. A physician who specializes in this field is called a neonatologist.
Patent ductus arteriosus (PDA)— An opening in the blood vessel that connects the aorta and the pulmonary artery. In full-term infants, this opening closes shortly after birth, but in premature infants, it may remain open and allow blood to collect in the infant's lungs.
Periventricular leukomalacia (PVL)— A brain disorder in which some of the white matter of the brain near the ventricles is softened because of the death of tissue in these areas.
Preeclampsia/eclampsia— Complications of pregnancy related to high blood pressure in a woman whose blood pressure was normal before pregnancy. Preeclampsia and eclampsia are common reasons for inducing premature labor.
Preterm— Another word for premature.
Respiratory distress syndrome (RDS)— A condition in which a premature infant lacks a sufficient amount of surfactant, a protective film that helps air sacs in the lungs to stay open.
Retinopathy of prematurity (ROP)— A condition in which the blood vessels in the retina of the eye display abnormal growth.
Surfactant— A protective film that helps air sacs in the lungs to stay open.
Tachypnea— Rapid breathing. Some premature infants develop rapid breathing for a few hours or days. This condition is known as transient tachypnea of the newborn (TTNB).
After discharge from the hospital, the infant's growth and development will be monitored by a pediatrician or family physician. This doctor will reinforce the parents' education about caring for their baby, review the hospital records, and give the baby its first immunizations (most can be given at the usual chronological age). Most doctors recommend office visits every one or two weeks until the infant has adapted satisfactorily to the home environment and is gaining an appropriate amount of weight.
Some of the risks and complications of premature delivery can be reduced if the mother receives good prenatal care, follows a healthy diet, avoids alcohol consumption, and refrains from cigarette smoking. In some cases of premature labor, the mother may be placed on bed rest or given drugs that can postpone labor for days or weeks, giving the fetus more time to develop before delivery. The physician or nurse-midwife may prescribe a steroid medication to be given to the mother if a premature birth is expected, to assist the baby's lung development.
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