About premature babies

PDF download is not available for Arabic and Urdu languages at this time. Please use the browser print function instead

Learn about premature babies. While premature babies can be born and develop normally, some may suffer growth and developmental problems later on in life.

Key points

  • Babies born before 37 weeks in the womb are considered premature.
  • Many, but not all, premature babies will require some form of specialized supportive medical care after birth.
  • There are multiple causes of premature birth, including conditions of the mother, conditions of the fetus, and conditions of the uterus or placenta.
  • There are many medical concerns associated with premature birth, including breathing problems, immature heart, gastrointestinal tract and kidneys.

Normally, a fetus spends 40 weeks growing and developing in the womb before birth and entry into the world. This length of time is called term or full-term. According to the World Health Organization (WHO), babies born before completing 37 weeks in the womb are defined as preterm or premature. Although preemies, as premature babies are sometimes called, can be perfectly healthy at birth and develop normally both in body and mind, almost all will require some form of specialized supportive medical care after birth, and some will have continuing problems that will affect their growth and development.

In the wrong place

Close-up of sleeping preemie

Often, when premature babies require medical support following birth, it is not necessarily that there is something “wrong” with the baby; in many cases, the problem is simply that they are in the wrong place. Instead of spending more time developing in their mother’s womb, they are out in the world, somewhat unprepared for an environment that requires a degree of independence. In the womb, the fetus is protected from temperature changes and physical contact, and is provided for in terms of nutrition and oxygen by the mother. This environment is vastly different from the outside world where a baby must breathe air to get oxygen into the blood, where they are susceptible to cold, and where they must use their digestive system to get the nutrition needed to grow and develop.

The gestation period, or the time the fetus spends in the womb, is essentially preparation for the outside world. It is the amount of time a fetus needs to develop the organs and mechanisms needed to deal with the challenges of living outside the womb. For this reason, the amount of time a baby has spent in the womb will largely determine the types and severities of problems that they have at birth and beyond.

Gestational age and birth weight

The length of time a baby has spent in the womb, or more specifically the number of completed weeks of pregnancy, is called the gestational age. In general, the earlier a baby is born, or the younger their gestational age, the more problems they are likely to have, and the less likely they are to go on to develop normally.

Additionally, the smaller the size of the baby at birth, or the lower the birth weight, the more likely they are to need support and the less likely they are to develop normally or perhaps survive at all. According to the WHO, babies born weighing less than 2500 grams or about 5 lbs 8 oz are defined as low-birth-weight babies.

Doctors always plot an infant’s gestational age and birth weight on a graph to see how close to normal an infant had been growing in the womb.

Small for gestational age

Although the size of a baby at birth is usually related to their gestational age, there are situations where babies are unusually small for their gestational age (SGA). Babies can be born SGA for any of several reasons:

  • a lack of nutrients and oxygen, which might be due to a poorly functioning placenta or the mother’s own illnesses
  • infections such as cytomegalovirus (CMV) or rubella
  • chromosomal or other congenital abnormalities
  • There are limits to how much the uterus can expand, which restricts fetal growth. Nature solves this problem by early delivery: twins tend to be born between 37 and 40 weeks, triplets between 34 and 37 weeks, and quadruplets between 28 and 32 weeks, or earlier.
  • the pregnant mother’s lifestyle: smoking cigarettes, taking drugs, and consuming alcohol during pregnancy are also risk factors for SGA births.

Although most babies are born with a weight that is considered appropriate for their gestational age (AGA), medical professionals are always on the lookout for SGA babies because they generally will require more interventions and a longer stay in the hospital’s neonatal intensive care unit (NICU). The SGA condition, especially combined with extreme prematurity, makes babies more at risk for certain medical conditions, especially neurological problems such as cerebral palsy.

What causes premature birth?

There are a number of known reasons why some babies are born premature. Some have to do with the condition of the mother, others with the condition of the fetus.

Maternal causes, or causes having to do with the general condition of the mother, include:

  • high blood pressure
  • infection
  • substance abuse
  • trauma
  • chronic illness

Premature birth can also come as a result of complications with either the uterus or the placenta. These include:

  • cervical incompetence, or an early opening of the cervix
  • malformation of the uterus
  • an excess of amniotic fluid, which can cause pressure in the uterus, resulting in early contractions and birth
  • malformations or early detachment of the placenta

Premature birth can also be the result of fetal causes, which include:

  • infection
  • fetal malformation
  • chromosomal abnormalities

Premature births also occur for unknown reasons. While some women know they may be at risk for a premature birth and can make preparations for it, most have an unexpected and unplanned premature birth.

The spectrum of prematurity

Based on their gestational age and birth weight, premature babies are placed into categories of mild, moderate and extreme prematurity.

  • Mild prematurityMildly premature baby lying in isolette

    Mild prematurity refers to babies who are born between 33 and 37 completed weeks gestational age and/or have a birth weight between 1500 and 2500 grams (about 3 lbs 5 oz to 5 lbs 8 oz).

  • Moderate prematurityModerately premature baby lying in an isolette

    Moderate prematurity refers to babies who are born between 28 and 32 completed weeks gestational age with a birth weight range between 1000 and 1500 grams (about 2 lbs 3 oz and 3 lbs 5 oz).

  • Extreme prematurityExtremely premature baby lying in isolette with parent holding hand

    Extreme prematurity refers to babies who are born before 28 completed weeks gestational age or have a birth weight of less than 1000 grams (about 2 lbs 3 oz).

In general, the younger the gestational age of the baby, the less prepared they are for the outside world. Mildly premature babies that have nearly completed the usual time of gestation in the womb often do not need much support at birth. They are quick to become independent and usually go on to develop into normal healthy children. By contrast, extremely premature babies, those who perhaps have spent only 25 weeks in the womb, require very intensive support at birth. They may have grave difficulties becoming independent, are at risk for major disabilities or may not survive at all.

Medical problems associated with premature birth

Although the majority of premature babies develop into healthy children, there are many different types of problems associated with prematurity. Even when everything is “normal”, prematurity itself has many challenges. Many of the body’s organs function differently in the womb than in the outside world and may not be fully ready to make the switch. A premature baby’s lungs, though on the road to developing normally in the womb, may not have developed enough to breathe without some form of assistance. Similarly, a premature baby will have an immature heart, gastrointestinal tract, kidneys, and so on, and may also require assistance in these areas for a time until they are more mature.

Additionally, premature babies are somewhat more fragile than term babies. Some parts of an immature body, for example the brain, are more vulnerable to injury as a result of their immaturity. Though this is not usually a problem while the baby is protected in the womb, a premature birth exposes the baby to more injury sources, including possibly the birth itself.

Last updated: October 31st 2009