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Premature Birth

  • Babies born before the beginning of the 37th week are considered premature
  • The causes that contribute to premature birth are not always clear
  • Premature babies receive specialized treatments to address serious health issues that may arise
  • Involves High-Risk Pregnancy Program and Maternal-Fetal Medicine

Premature Birth


A pregnancy that runs its full term lasts 40 weeks, calculated from the start of the mother’s last menstrual cycle. Babies born before the beginning of the 37th week are considered premature. Crucial development continues to occur those last few weeks of pregnancy, including lung growth that ensures adequate breathing and maturation of the digestive system, to ensure proper digestion of nutrients.

Babies born prematurely may face medical and developmental complications but, with time and care, usually  grow into happy, healthy children, says Mark R. Mercurio, MD, MA, chief of Yale Medicine Neonatal-Perinatal Medicine. 

“Some children born prematurely may have troubles, ranging from mild to severe,” he says. “But most are not going to have severe long-term disabilities. The vast majority of babies born under 28 weeks now leave our unit alive, and most will not have severe long term problems associated with preterm birth.”

What are some risk factors that can lead to premature birth?

The causes that contribute to premature birth are not always clear says Mark R. Mercurio, MD, MA, chief of Yale Medicine Neonatal-Perinatal. It happens to women of all ages, races and ethnic groups. “What makes a woman go into labor on Tuesday instead of Wednesday is not perfectly understood.”

Still, there are some specific factors that may play a role. These include previously having had a preterm baby, multiples (twins or more), infection, smoking or use of illicit drugs, poor nutrition, and poor health, including dental hygiene.

“We think that inconsistent or poor prenatal care makes it more likely,” Dr. Mercurio says. “So we can try to ensure overall good maternal health, which starts with a woman building an ongoing relationship with her obstetrician.”

What complications can develop because of premature birth?

Complications vary and tend to be more severe the earlier a baby. Because a premature baby doesn’t have the chance to develop sufficient nutritional stores and strong organs and systems before birth, the baby must rely on supports provided in the neonatal intensive care unit (NICU), which have to take over where the gestation process ended.

“When a baby is born prematurely, what we try to do in some ways is largely mimic what happens in utero," says Dr. Mercurio, noting that some problems, such as immature lungs, require special technology. The needs of each premature baby are unique but common issues faced by babies in the NICU include: 

  • Lungs. One of the biggest troubles that premature babies face is with breathing, because their lungs have not fully developed. A concern is that respiratory distress syndrome (RDS), a disorder in which the lungs lack a fluid lining called surfactant that prevents lung collapse, can develop. Some who have RSD (especially those born at the earliest gestational ages) go on to develop a more chronic condition called bronchopulmonary dysplasia (BPD); they may require longer help with breathing.
  • Heart. Like the lungs, the heart may not be fully mature. Babies born early can face a variety of problems, including patent ductus arteriosus, in which a blood vessel near the heart does not have the opportunity to close normally. This can lead to congestive heart failure.
  • Blood. Babies born very early often lack the ability to produce red blood cells in adequate numbers, and necessary blood tests can further deplete their already less than optimal blood supply. Thus, significant anemia can develop in the early days and weeks of life.
  • Brain. Because of the immaturity of the brain’s structure, there’s a possibility of a germinal matrix hemorrhage (bleeding in the brain). Many also experience apnea of prematurity, which essentially means that the child sometimes stops breathing, especially while asleep. This is due primarily to brain immaturity, and may be another reason a baby might require help with breathing for a short time. All babies in the NICU are monitored constantly for apnea and outgrow it well before they are discharged home.
  • Body temperature. The final weeks of pregnancy are when babies develop much of their fat and muscle, which help to regulate temperature once they are born. Without that mass, a premature baby may need help keeping warm.
  • Nutrition. A premature baby may not be able to digest food properly, may not have developed the instincts to suck and swallow, and may not be able to maintain a normal blood sugar (as a full term baby would).
  • Immunity. An immature immune system can predispose babies born early to infection.

In the long term, premature babies can also be at a greater risk for complications including cognitive delays, vision and hearing problems, behavioral issues and sometimes cerebral palsy.

What treatments may a premature baby receive?

  • Lungs. RDS is often treated with various levels of respiratory support. Options include extra oxygen, the use of a mechanical ventilator, and administration of a surfactant solution to the lungs. Babies who develop BPD might require medications to help the lungs function better. 
  • Heart. If the baby is suffering from patent ductus arteriosus, and the blood vessel doesn’t close on its own, medication may be prescribed that helps spur this development. Surgical treatment can be an option too, though that's rarely a necessity.
  • Blood. Premature babies commonly receive one or more transfusions in the early weeks of life. After several weeks, however, production of the blood cells picks up, and this problem is usually resolved well before discharge home.
  • Brain: Most premature babies will receive at least one ultrasound of the brain over the course of their hospitalization, and possibly magnetic resonance imaging (MRI), to monitor for germinal matrix hemorrhage and other brain problems. Those who require it will be prescribed indomethacin, an anti-inflammatory medication that reduces the likelihood of a brain bleed. Apnea of prematurity is often treated with caffeine, which stimulates the central nervous system. “If a baby still has problems with breathing, we will sometimes use respiratory support,” says Dr. Mercurio. 
  • Body temperature. If a premature baby needs help staying warm, the baby may be kept in a temperature-regulated box (often called an incubator) until he or she is able to maintain a normal temperature without help.
  • Nutrition. To ensure the baby’s glucose levels are adequate, particularly in the first few days of life, the doctor may insert a catheter that infuses sugar water into the baby’s umbilical artery or umbilical vein. Doctors may also provide the needed fluids and nutrition intravenously via a special formulation known as total parental nutrition (TPN). For several weeks, some babies are fed through a small flexible tube, inserted into the nose or mouth and threaded into the stomach. This will gradually be transitioned to normal feeding as the baby matures.
  • Immunity. Because the immune system is usually compromised, special precautions are often taken to minimize the risk of infection. Premature babies are closely monitored for early signs of trouble, which often require treatment with antibiotics.

What happens after a premature baby leaves the hospital?

The timing of a premature baby’s departure from the intensive care unit will vary from child to child. But a good rule of thumb is that the baby will go home around the due date.

“That’s easy to remember, and it gives parents a very rough approximate target,” says Dr. Mercurio. “If things go well, most babies go home between 36 and 40 weeks.”

In order to leave the NICU, the baby needs to have developed some important functions.

“They have to be able to breathe without us reminding them,” says Dr. Mercurio. Babies also need to  be able to keep warm, to eat without choking, and take enough milk so they can grow without needing a tube. "All of that tends to come together by around 37 weeks," he notes.

After this milestone, babies can usually leave intensive care and go home to to continue developing like those who went to full term in the womb. But the premature baby will require more careful monitoring and parents can usually expect to have frequent visits to the pediatrician.  

“Beyond making sure the baby is simply gaining weight well, the pediatrician may see them him or her more often than other kids because the child’s at an increased risk for a number of things, including developmental delay,” Dr. Mercurio says. “Some kids need more interventions early on to help them develop – physical therapy, for example.”

What makes Yale Medicine’s approach to treating premature birth unique?

Yale Medicine has been recognized as a national leader in the prevention of hospital-acquired infections in the NICU, having won multiple awards for this work. The medical director of the NICU, Matthew Bizzarro, MD, is a widely sought-after authority in infection control.

The care of premature babies after discharge at Yale Medicine is coordinated in the NICU GRAD (“great results after discharge”) Program, directed by Angela Montgomery, MD, MSEd. Dr. Montgomery has training and experience as a teacher, pediatrician and neonatologist, and leads a multidisciplinary team of doctors, nurses, physical therapists and others that follows the progress of the NICU “graduates,” to ensure that all needed services are in place in order to optimize long-term outcomes.

Yale Medicine carries out several clinical research projects in an ongoing effort to improve the treatments available to premature babies. These efforts are led by Richard Ehrenkranz, MD, an internationally known expert in clinical research in premature babies, particularly in the areas of nutrition and lung disease.

One aspect of premature birth that Yale Medicine examines and explores is the ethics of treatment and the role of the parents and staff in difficult decisions.

“One of the differences in the care of preterm babies is the decision-making process, particularly when it comes to the tiniest ones, at 22, 23, 24 weeks,” says Dr. Mercurio. “How hard should we try to keep these children alive, given the low chance of survival for some, given the high risk of brain injury for some? What should and shouldn’t be done for these children? This is a source of tremendous discussion within the pediatric profession and within society at large.”

Dr. Mercurio, in particular, and his colleagues, have taken a leading role in those discussions, speaking nationally and internationally, and developing a fellowship at Yale Medicine that enables top doctors to come and learn, and then lead the way ethically at their institutions. “So much of the hardest work in this is related to the ethical decision making,” Dr. Mercurio says. “It’s not always clear what the right thing to do is.”

Intertwined with the ethics, he says, is how doctors and parents come to these very difficult decisions.

“Most of these preterm babies are going to have a good outcome and live a long and happy life,” he says. “But for some of these parents, even for the parents who have a relatively easy road, a relatively easy road in the newborn ICU is still a very difficult ordeal for parents. Helping them through that is a big part of what we do.”