The average length of a normal pregnancy is 40 weeks (280 days) from the date of conception. Infants born before 37 weeks gestation are considered premature and may be at risk for complications.
More than one out of every ten infants born in the United States is born prematurely. Advances in medical technology have made it possible for infants born as young as 23 weeks gestational age (17 weeks premature) to survive. These premature infants, however, are at higher risk for death or serious complications, which include heart defects, respiratory problems, blindness, and brain damage.
According to the March of Dimes Foundation, there were 480,812 births in the United States in 2002 that occurred before 37 weeks gestation. This number represents 12.1 percent of live births that year. In an average week, approximately 9,200 infants are born prematurely, and approximately 1,500 are born before 32 weeks gestation. Black infants have the highest prematurity rate with 17.6 percent of live births; Native American (12.9%); Hispanic infants (11.4%); white infants (10.7%); and Asian infants (10.2%). Mothers younger than 20 years of age or older than 35 years of age have higher rates of preterm delivery.
Causes and symptoms
The birth of a premature baby can be brought on by several different factors, including the following:
- premature labor
- placental abruption, in which the placenta detaches from the uterus
- placenta previa, in which the placenta grows too low in the uterus
- premature rupture of membranes, in which the amniotic sac is torn, causing the amniotic fluid to leak out
- incompetent cervix, in which the cervix opens too soon
- maternal toxemia or preeclampsia
Prematurity is much more common in pregnancy of multiples and for mothers who have a history of miscarriages or prior premature birth. Another identifiable cause of prematurity is drug abuse (e.g. cocaine) by the mother.
Infants born prematurely may experience major complications due to their low birth weight and the immaturity of their organ systems. Some of the common problems among premature infants are jaundice (yellow discoloration of the skin and whites of the eyes), apnea (a long pause in breathing), and inability to breast or bottle feed. Body temperature, blood pressure, and heart rate may be difficult to regulate in premature infants. The lungs, digestive system, and nervous system (including the brain) are underdeveloped in premature babies and are particularly vulnerable to complications.
Respiratory distress syndrome (RDS) is the most common problem in premature infants. Babies born too soon have immature lungs that have not developed surfactant, a protective film that helps air sacs in the lungs to stay open. With RDS, breathing is rapid and the center of the chest and rib cage pull inward with each breath. Extra oxygen can be supplied to the infant through tubes that fit into the nostrils of the nose or by placing the baby under an oxygen hood. In more serious cases, the baby may have to have a breathing tube inserted and receive air from a respirator or ventilator. A surfactant drug can be given in some cases. Extra oxygen may be needed for a few days or weeks. Bronchopulmonary dysplasia is the development of scar tissue in the lungs and can occur in severe cases of RDS.
Necrotizing enterocolitis (NEC) is another complication of prematurity. In this condition, part of the baby's intestine is destroyed as a result of bacterial infection. In cases where only the innermost lining of the bowel dies, the infant's body can regenerate it over time; however, if the full thickness of a portion dies, it must be removed surgically and an opening (ostomy) must be made for the passage of wastes until the infant is healthy enough for the remaining ends to be sewn together. Because NEC is potentially fatal, doctors are quick to respond to its symptoms, which include lethargy, vomiting , a swollen and/or red abdomen, fever , and blood in the stool. Measures include taking the infant off mouth feedings and feeding him or her intravenously, administering antibiotics , and removing air and fluids from the digestive tract via a nasal tube. Approximately 70 percent of NEC cases can be successfully treated without surgery.
Intraventricular hemorrhage (IVH) is another serious complication of prematurity. It is a condition in which immature and fragile blood vessels within the brain burst and bleed into the hollow chambers (ventricles) normally reserved for cerebrospinal fluid and into the tissue surrounding them. Physicians grade the severity of IVH according to a scale of I through IV, with I being bleeding confined to a small area around the burst vessels and IV being an extensive collection of blood in the ventricles and in the brain tissue itself. Grades I and II are not uncommon, and the baby's body usually reabsorbs the blood with no ill effects. However, more severe IVH can result in hydrocephalus , a potentially fatal condition in which too much fluid collects in the ventricles, exerting increased pressure on the brain and causing the baby's head to expand abnormally. To drain fluid and relieve pressure on the brain, doctors either perform lumbar punctures, a procedure in which a needle is inserted into the spinal canal to drain fluid; install a reservoir, a tube that drains fluid from a ventricle and into an artificial chamber under or on top of the scalp; or install a ventricular shunt, a tube that drains fluid from the ventricles and into the abdomen, where it is reabsorbed by the body. Infants who are at high risk for IVH usually have an ultrasound taken of their brain in the first week after birth, followed by others if bleeding is detected. IVH cannot be prevented; however, close monitoring can ensure that procedures to reduce fluid in the brain are implemented quickly to minimize possible damage.
Apnea of prematurity is a condition in which the infant stops breathing for periods lasting up to 20 seconds. It is often associated with a slowing of the heart rate. The baby may become pale, or the skin color may change to a blue or purplish hue. Apnea occurs most commonly when the infant is asleep. Infants with serious apnea may need medications to stimulate breathing or oxygen through a tube inserted in the nose. Some infants may be placed on a ventilator or respirator with a breathing tube inserted into the airway. As the baby gets older, and the lungs and brain tissues mature, the breathing usually becomes more regular. A group of researchers in Cleveland reported in 2003, however, that children who were born prematurely are three to five times more likely to develop sleep-disordered breathing by age 10 than children who were full-term babies.
As the fetus develops, it receives the oxygen it needs from the mother's blood system. Most of the blood in the infant's system bypasses the lungs. Once the baby is born, its own blood must start pumping through the lungs to get oxygen. Normally, this bypass duct closes within the first few hours or days after birth. If it does not close, the baby may have trouble getting enough oxygen on its own. Patent ductus arteriosus is a condition in which the duct that channels blood between two main arteries does not close after the baby is born. In some cases, a drug called indomethacin can be given to close the duct. Surgery may be required if the duct does not close on its own as the baby develops.
Retinopathy of prematurity is a condition in which the blood vessels in the baby's eyes do not develop normally, and can, in some cases, result in blindness. Premature infants are also more susceptible to infections. They are born with fewer antibodies, which are necessary to fight off infections.
When to call the doctor
In some cases, healthcare professionals are able to stop or delay premature labor if treated early enough. A pregnant woman should contact her healthcare provider if she observes any of the signs of premature labor, including the following:
- contractions closer than 10 minutes apart
- leaking fluid or bleeding from the vagina
- menstrual-like cramps
- low, dull backache
- pelvic pressure
Many of the problems associated with prematurity depend on how early the baby is born and how much it weighs at birth. The most accurate way of determining the gestational age of an infant in utero is calculating from a known date of conception or using ultrasound imaging to observe development. When a baby is born, doctors can use the Dubowitz exam to estimate gestational age. This standardized test scores responses to 33 specific neurological stimuli to estimate the infant's neural development. Once the baby's gestational age and weight are determined, further tests and electronic fetal monitoring may need to be used to diagnose problems or to track the baby's condition. A blood pressure monitor may be wrapped around the arm or leg. Several types of monitors can be taped to the skin. 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. X-ray or ultrasound imaging may be used to examine the heart, lungs, and other internal organs.
Treatment depends on the types of complications that are present. It is not unusual for premature infants to be placed in heat-controlled units (incubators) to maintain their temperature. Infants who are having trouble breathing on their own may need oxygen either pumped into the incubator, administered through small tubes placed in their nostrils, or through a respirator or ventilator, which pumps air into a breathing tube inserted into the airway. They may require fluids and nutrients to be administered through an intravenous line, in which a small needle is inserted into a vein in the hand, foot, arm, leg, or scalp. If the baby needs drugs or medications, these may also be administered through the intravenous line. Another type of line may be inserted into the baby's umbilical cord. This can be used to draw blood samples or to administer medications or nutrients. If heart rate is irregular, the baby may have heart monitor leads taped to the chest. Many premature infants require time and support with breathing and feeding until they mature enough to breathe and eat unassisted. Depending on the complications, the baby may require drugs or surgery.
Research has shown that the risks of massaging preterm infants are minimal and that infants benefit from improved developmental scores, more rapid weight gain, and earlier discharge from the hospital. An additional benefit of massage therapy is closer bonding between the parents and their newborn child. Another method, called kangaroo care, entails placing a medically stable, diaper-clad premature infant on a parent's chest for periods of time so that the parent and child are touching skin-to-skin. A 2002 study published in Pediatrics found that both the parent and infant benefited from the practice: mothers reported lower rates of depression and more sensitivity to the infant's needs, and the infants showed improved cognitive and motor development.
If a premature infant is unable to nurse at the breast or drink from a bottle, fluids and nutrients may be administered intravenously or with a tube in the nose or mouth that empties into the stomach (called gavage feeding). Even if a baby is unable to feed at the breast, a mother may pump her breast milk to be given to the infant via gavage feeding. Once the infant learns to suck and swallow effectively, breast or bottle feedings can commence.
Advances in medical care have made it possible for many premature infants to survive and develop normally. Whether a premature infant survives, however, is still intimately tied to his or her gestational age:
- 21 weeks or less: 0 percent survival rate
- 22 weeks: 0 to 10 percent survival rate
- 23 weeks: 10 to 35 percent survival rate
- 24 weeks: 40 to 70 percent survival rate
- 25 weeks: 50 to 80 percent survival rate
- 26 weeks: 80 to 90 percent survival rate
- 27 weeks: greater than 90 percent survival rate
Physicians cannot predict long-term complications of prematurity; some consequences may not become evident until the child is school age. Minor disabilities like learning problems, poor coordination, or short attention span may be the result of premature birth but can be overcome with early intervention. The risks of serious long-term complications depend on many factors, including how premature the infant was at birth, the weight at birth, and the presence or absence of breathing problems. Gender is an associated factor: a Swedish study published in 2003 found that boys are at greater risk of death or serious long-term consequences of prematurity than girls. For example, 60 percent of boys born at 24 weeks' gestation die compared to 38 percent mortality for girls. The development of infection or the presence of a birth defect can also affect long-term prognosis. Infants who have infections in prematurity and very low birth weight are at risk for later disorders of the nervous system; a study done at Johns Hopkins reported that 77 out of a group of 213 premature infants developed neurologic disorders. Severe disabilities such as brain damage, blindness, and chronic lung problems are possible and may require ongoing care.
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 or drug 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 stop labor contractions for days or weeks, giving the developing infant more time to develop before delivery. The physician may prescribe a steroid medication to be given to the mother before the delivery to help speed up the baby's lung development. The availability of a neonatal intensive care unit (NICU), a special hospital unit equipped and trained to deal with premature infants, can also increase an infant's chances of survival.
A new medication may help to prevent spontaneous premature births. Researchers at Wake Forest University reported in June 2003 that a drug known as 17 alpha-hydroxyprogesterone caproate reduced the number of premature births in a group of women who received weekly injections of the drug compared to a placebo group and lowered the rates of necrotizing enterocolitis, intraventricular hemorrhage, and need for supplemental oxygen in their infants.
Poor nutrition during pregnancy may lead to an increased risk of premature delivery. Research supported by the U.S. Public Health Service during the 1990s found that an inadequate diet during pregnancy was associated with premature rupture of amniotic sac membranes and premature birth. A well-balanced diet rich in nutrients such as folic acid , calcium, iron, zinc, vitamin D, and the B vitamins is recommended for pregnant women. Mothers are recommended to eat approximately 300 additional calories a day (above and beyond a normal non-pregnancy diet) to support the fetus's growth and development.
Parents are often overwhelmed at the prospect of caring for a premature baby. Parents of infants being cared for in the NICU are often recommended to feed, change, and hold their child as long as he or she is medically stable. After the infant leaves the hospital, the parents can seek support from many professional and parent-to-parent resources, including books, web sites, support groups, and national organizations.
Apnea —The temporary absence of breathing. Sleep apnea consists of repeated episodes of temporary suspension of breathing during sleep.
Dubowitz exam —Standardized test that scores responses to 33 specific neurological stimuli to estimate an infant's neural development and, hence, gestational age.
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.
Jaundice —A condition in which the skin and whites of the eyes take on a yellowish color due to an increase of bilirubin (a compound produced by the liver) in the blood. Also called icterus.
Necrotizing enterocolitis —A serious bacterial infection of the intestine that occurs primarily in sick or premature newborn infants. It can cause death of intestinal tissue (necrosis) and may progress to blood poisoning (septicemia).
Respiratory distress syndrome (RDS) —Also known as hyaline membrane disease, this is a condition of premature infants in which the lungs are imperfectly expanded due to a lack of a substance (surfactant) on the lungs that reduces tension.
Retinopathy of prematurity —A condition in which the blood vessels in a premature infant's eyes do not develop normally. It can, in some cases, result in blindness.
Surfactant —A protective film secreted by the alveoli in the lungs that reduces the surface tension of lung fluids, allowing gas exchange and helping maintain the elasticity of lung tissue. Surfactant is normally produced in the fetal lungs in the last months of pregnancy, which helps the air sacs to open up at the time of birth so that the newborn infant can breathe freely. Premature infants may lack surfactant and are more susceptible to respiratory problems without it.
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Altha Roberts Edgren Rebecca J. Frey, PhD Stephanie Dionne Sherk