Overview

A premature birth is a birth that takes place more than three weeks before the baby is due. In other words, a premature birth is one that occurs before the start of the 37th week of pregnancy. Normally, a pregnancy usually lasts about 40 weeks.

Premature birth gives the baby less time to develop in the womb. Premature babies, especially those born earliest, often have complicated medical problems.

Depending on how early a baby is born, he or she may be:

  • Late preterm, born between 34 and 36 weeks of pregnancy
  • Moderately preterm, born between 32 and 34 weeks of pregnancy
  • Very preterm, born at less than 32 weeks of pregnancy
  • Extremely preterm, born at or before 25 weeks of pregnancy

Most premature births occur in the late preterm stage.

Symptoms

A premature birth means that your baby hasn't had the usual amount of time to develop in the womb before needing to adapt to life outside the womb.

The signs that a baby's gestation has been cut short include:

  • Small size, with a disproportionately large head
  • Sharper looking, less rounded features than a full-term baby's features, due to a lack of fat stores
  • Fine hair (lanugo) covering much of the body
  • Low body temperature, especially immediately after birth in the delivery room, due to a lack of stored body fat
  • Labored breathing or respiratory distress
  • Lack of reflexes for sucking and swallowing, leading to feeding difficulties

The following tables show the median birth weight, length and head circumference of premature babies at different gestational ages for each sex.

Weight, length and head circumference by gestational age for boys
Gestational age Weight Length Head circumference
40 weeks 7 lbs., 15 oz.
(3.6 kg)
20 in. (51 cm) 13.8 in. (35 cm)
35 weeks 5 lbs., 8 oz.
(2.5 kg)
18.1 in. (46 cm) 12.6 in. (32 cm)
32 weeks 3 lbs., 15.5 oz.
(1.8 kg)
16.5 in. (42 cm) 11.6 in. (29.5 cm)
28 weeks 2 lbs., 6.8 oz.
(1.1 kg)
14.4 in. (36.5 cm) 10.2 in. (26 cm)
24 weeks 1 lb., 6.9 oz.
(0.65 kg)
12.2 in. (31 cm) 8.7 in. (22 cm)
Weight, length and head circumference by gestational age for girls
Gestational age Weight Length Head circumference
40 weeks 7 lbs., 7.9 oz.
(3.4 kg)
20 in. (51 cm) 13.8 in. (35 cm)
35 weeks 5 lbs., 4.7 oz.
(2.4 kg)
17.7 in. (45 cm) 12.4 in. (31.5 cm)
32 weeks 3 lbs., 12 oz.
(1.7 kg)
16.5 in. (42 cm) 11.4 in. (29 cm)
28 weeks 2 lbs., 3.3 oz.
(1.0 kg)
14.1 in. (36 cm) 9.8 in. (25 cm)
24 weeks 1 lb., 5.2 oz.
(0.60 kg)
12.6 in. (32 cm) 8.3 in. (21 cm)

Premature babies can quickly develop serious complications, such as infection in the bloodstream (sepsis) and respiratory distress syndrome.

When to see a doctor

You'll be seeing members of the neonatal intensive care unit (NICU) often. Doctors often visit NICUs at a similar time each day. But, don't hesitate to ask questions even when there's not a regularly scheduled meeting, especially if your baby seems listless, has poor color, or refuses the bottle or breast after trouble-free feedings.

Risk factors

Often, the specific cause of premature birth isn't clear. Many factors may increase the risk of premature birth, however, including:

  • Having a previous premature birth
  • Pregnancy with twins, triplets or other multiples
  • An interval of less than six months between pregnancies
  • Conceiving through in vitro fertilization
  • Problems with the uterus, cervix or placenta
  • Smoking cigarettes or using illicit drugs
  • Poor nutrition
  • Not gaining enough weight during pregnancy
  • Some infections, particularly of the amniotic fluid and lower genital tract
  • Some chronic conditions, such as high blood pressure and diabetes
  • Being underweight or overweight before pregnancy
  • Stressful life events, such as the death of a loved one or domestic violence
  • Multiple miscarriages or abortions
  • Physical injury or trauma

For unknown reasons, black women are more likely to experience premature birth than are women of other races. But premature birth can happen to anyone. In fact, many women who have a premature birth have no known risk factors.

Complications

While not all premature babies experience complications, being born too early can cause short-term and long-term health problems for preemies. Generally, the earlier a baby is born, the higher the risk of complications. Birth weight plays an important role, too.

Some problems may be apparent at birth, while others may not develop until later.

Short-term complications

In the first weeks, the complications of premature birth may include:

  • Breathing problems. A premature baby may have trouble breathing due to an immature respiratory system. If the baby's lungs lack surfactant — a substance that allows the lungs to expand — he or she may develop respiratory distress syndrome because the lungs can't expand and contract normally.

    Preemies may also develop chronic lung disease known as bronchopulmonary dysplasia. In addition, some preemies experience prolonged pauses in their breathing, known as apnea.

  • Heart problems. The most common heart problems premature babies experience are patent ductus arteriosus (PDA) and low blood pressure (hypotension). PDA is a persistent opening between two major blood vessels leading from the heart.

    While this heart defect often closes on its own, left untreated it can cause too much blood to flow through the heart and cause heart failure as well as other complications. Low blood pressure may require adjustments in intravenous fluids, medicines and sometimes blood transfusions.

  • Brain problems. The earlier a baby is born, the greater the risk of bleeding in the brain, known as an intraventricular hemorrhage. Most hemorrhages are mild and resolve with little short-term impact. But some babies may have larger brain bleeding which causes permanent brain injury.

    Larger brain bleeds may lead to fluid accumulation in the brain (hydrocephalus) over a number of weeks. Some babies who develop hydrocephalus will require an operation to relieve the fluid accumulation.

  • Temperature control problems. Premature babies can lose body heat rapidly; they don't have the stored body fat of a full-term infant and they can't generate enough heat to counteract what's lost through the surface of their bodies. If body temperature dips too low, hypothermia can result.

    Hypothermia in a preemie can lead to breathing problems and low blood sugar levels. In addition, a preemie may use up all of the energy gained from feedings just to stay warm, not to grow bigger. That's why smaller preemies require additional heat from a warmer or an incubator until they're larger and able to maintain body temperature without assistance.

  • Gastrointestinal problems. Preemies are more likely to have immature gastrointestinal systems, leaving them predisposed to complications such as necrotizing enterocolitis (NEC). This potentially serious condition, in which the cells lining the bowel wall are injured, can occur in premature babies after they start feeding. Premature babies who receive only breast milk have a much lower risk of developing NEC.
  • Blood problems. Preemies are at risk of blood problems such as anemia and infant jaundice. Anemia is a common condition in which the body doesn't have enough red blood cells. While all newborns experience a slow drop in red blood cell count during the first months of life, the decrease may be greater in preemies, especially if your baby has a lot of blood taken for lab tests.

    Infant jaundice is a yellow discoloration in a newborn baby's skin and eyes that occurs because the baby's blood contains an excess of a yellow-colored pigment from the liver or red blood cells (bilirubin).

  • Metabolism problems. Premature babies often have problems with their metabolism. Some preemies may develop an abnormally low level of blood sugar (hypoglycemia). This can happen because preemies typically have smaller stores of glycogen (stored glucose) than do full-term babies and because preemies' immature livers have trouble converting stored glycogen into glucose.
  • Immune system problems. An underdeveloped immune system, common in premature babies, can lead to infection. Infection in a premature baby can quickly spread to the bloodstream causing sepsis, a life-threatening complication.

Long-term complications

In the long term, premature birth may lead to these complications:

  • Cerebral palsy. Cerebral palsy is a disorder of movement, muscle tone or posture that can be caused by infection, inadequate blood flow or injury to a preemie's developing brain either during pregnancy or while the baby is still young and immature.
  • Impaired cognitive skills. Premature babies are more likely to lag behind their full-term counterparts on various developmental milestones. Upon school age, a child who was born prematurely might be more likely to have learning disabilities.
  • Vision problems. Premature infants may develop retinopathy of prematurity, a disease that occurs when blood vessels swell and overgrow in the light-sensitive layer of nerves at the back of the eye (retina). Sometimes the abnormal retinal vessels gradually scar the retina, pulling it out of position. When the retina is pulled away from the back of the eye, it's called retinal detachment, a condition that, if undetected, can impair vision and cause blindness.
  • Hearing problems. Premature babies are at increased risk of some degree of hearing loss. All babies will have their hearing checked before going home.
  • Dental problems. Preemies who have been critically ill are at increased risk of developing dental problems, such as delayed tooth eruption, tooth discoloration and improperly aligned teeth.
  • Behavioral and psychological problems. Children who experienced premature birth may be more likely than full-term infants to have certain behavioral or psychological problems, such as attention-deficit/hyperactivity disorder (ADHD). However, more recent research suggests that — at least for late preterm babies — the risk of ADHD may be the same as it is for children who were born at full term.
  • Chronic health issues. Premature babies are more likely to have chronic health issues — some of which may require hospital care — than are full-term infants. Infections, asthma and feeding problems are more likely to develop or persist. Premature infants are also at increased risk of sudden infant death syndrome (SIDS).

Prevention

Although the exact cause of preterm birth is often unknown, there are some things that can be done to help women — especially those who have an increased risk — to reduce their risk of preterm birth, including:

  • Progesterone supplements. Women who have a history of preterm birth, a short cervix or both factors may be able to reduce the risk of preterm birth with progesterone supplementation.
  • Cervical cerclage. This is a surgical procedure that helps prevent premature birth in women pregnant with only one baby who have a history of premature birth or who have a short cervix or both.

    During this procedure, the cervix is stitched closed with strong sutures that may provide extra support to the uterus. The sutures are removed when it's time to deliver the baby. Ask your doctor if you need to avoid vigorous activity during the remainder of your pregnancy.

Although not as well-studied as progesterone supplements and cerclage, there's also some evidence that the following things may help reduce the risk of premature birth:

  • Eat a healthy diet even before you get pregnant. Although it's no guarantee of a full-term pregnancy, eating a healthy diet containing several servings of protein-rich foods, fruits and whole grains before you get pregnant may reduce your risk of premature birth. But, a diet high in fat, sugar and processed foods, such as chips and white bread, before pregnancy has been linked to a higher risk of premature birth.
  • Calcium supplements. Taking 1,000 milligrams (mg) or more of calcium daily may lower your risk of preeclampsia — a serious complication of pregnancy — and preterm birth, especially for women who normally consume low amounts of calcium in their diets.
  • Daily low-dose aspirin. Beginning late in the first trimester, taking a low-dose aspirin — 60 to 80 mg — daily appears to help certain women reduce the risk of premature birth. Women who may benefit from low-dose aspirin therapy include those with a history of preterm birth or preeclampsia, as well as women with chronic high blood pressure.
  • Cervical pessary. Women pregnant with just one baby who have a short cervix may reduce their risk of premature birth with a cervical pessary. A pessary is a silicone device that fits around the cervix and may help support the uterus.
  • Reduce chemical exposure. Exposure to phthalates — chemicals used to manufacture many products such as plastic, canned foods, cosmetics, nail polish and hairspray — is associated with a higher risk of premature birth. It isn't yet known if reducing exposure to these products would reduce the risk of premature birth, but it's a good idea to limit your exposure whenever you can.
Nov. 27, 2014
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