Your baby may have very mild symptoms of premature birth, or may have more-obvious complications.

Some signs of prematurity include the following:

• 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.

Often, the specific cause of premature birth isn't clear. However, there are known risk factors of premature delivery, 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
• 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.

While not all premature babies experience complications, being born too early can cause short-term and long-term health problems. 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.

Premature babies may also develop a lung disorder known as bronchopulmonary dysplasia. In addition, some preterm babies may 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 the aorta and pulmonary artery. While this heart defect often closes on its own, left untreated it can lead to a heart murmur, 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 that causes permanent brain injury.
• 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, an abnormally low core body temperature (hypothermia) can result.

Hypothermia in a premature baby can lead to breathing problems and low blood sugar levels. In addition, a premature infant may use up all of the energy gained from feedings just to stay warm. That's why smaller premature infants require additional heat from a warmer or an incubator until they're larger and able to maintain body temperature without assistance.

• Gastrointestinal problems. Premature infants are more likely to have immature gastrointestinal systems, resulting in 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. Premature babies are at risk of blood problems such as anemia and newborn 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 premature babies.

Newborn jaundice is a yellow discoloration in a baby's skin and eyes that occurs because the baby's blood contains excess bilirubin, a yellow-colored substance, from the liver or red blood cells. While there are many causes of jaundice, it is more common in preterm babies.

• Metabolism problems. Premature babies often have problems with their metabolism. Some premature babies may develop an abnormally low level of blood sugar (hypoglycemia). This can happen because premature infants typically have smaller stores of stored glucose than do full-term babies. Premature babies also have more difficulty converting their stored glucose into more-usable, active forms of glucose.
• Immune system problems. An underdeveloped immune system, common in premature babies, can lead to a higher risk of infection. Infection in a premature baby can quickly spread to the bloodstream, causing sepsis, an infection that spreads to the bloodstream.

Long-term complications

In the long term, premature birth may lead to the following 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 newborn's developing brain either early during pregnancy or while the baby is still young and immature.
• Impaired learning. 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. Premature infants 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, as well as developmental delays.
• 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).