Prematurity: Overview

Normally, a fetus spends 40 weeks growing and developing before birth. This length of time is called term or full term. According to the World Health Organization (WHO), babies born before completing 37 weeks’ gestation are defined as preterm or premature. Although preemies, as premature babies are sometimes called, can be perfectly healthy at birth and develop well, 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.

Prematurity

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, they are out in the world, somewhat unprepared for an environment that requires a degree of independence. During pregnancy, the fetus is protected from temperature changes and physical contact and receives nutrition and oxygen through the placenta. 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. For this reason, the length of a fetus’s gestation period will largely determine the types and severities of problems that they have at birth and beyond.

Gestational age and birth weight

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 medical needs they are likely to have and the longer they may need to stay in hospital.

Additionally, the smaller the size of the baby at birth, or the lower the birth weight, the more likely they are to need support. According to the WHO, babies born weighing less than 2,500 grams or about 5 pounds 8 ounces are defined as low-birth-weight babies.

An infant’s gestational age and birth weight are always plotted on a graph to see how well a fetus has been growing prior to birth.

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 a pregnant individual’s medical condition
• infections such as cytomegalovirus (CMV) or rubella
• chromosomal or other congenital abnormalities
• restrictions in the ability of the uterus to expand
• other risk factors: cigarette smoking, substance use and alcohol consumption during pregnancy all increase the likelihood of an SGA birth

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).

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 pregnant individual, while others relate to the condition of the fetus.

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

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

Premature birth can also be due to complications of the uterus or the placenta. These include:

• cervical insufficiency, 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
• congenital anomalies
• twin, triplet or other multiple pregnancies

Premature births also occur for unknown reasons. While some pregnant individuals 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, premature babies are placed into categories: late preterm, moderate preterm, very preterm and extremely preterm.

• Late preterm

  

  Late preterm babies are born between 34 and 36 weeks’ gestation.

• Moderately preterm

  

  Moderately preterm babies are born between 32 and 34 weeks’ gestation.

• Very preterm

  

  Very preterm babies are born between 28 and 32 weeks’ gestation.

• Extremely preterm

  

  Extremely preterm babies are born at less than 28 weeks’ gestation.

Prematurity is often also categorized by birth weight:

In general, the younger the gestational age of the baby and the lower the birth weight, the less prepared they are for the outside world. Late preterm babies often do not need much support at birth. They are quick to become independent and can often be discharged within a few days. By contrast, extremely preterm babies require very intensive support after birth. They may have grave difficulties becoming independent and are at higher risk for major disabilities.

Medical conditions associated with premature birth

Although the majority of premature babies develop into healthy children, there are many different types of problems associated with prematurity. Many of the body’s organs are not fully developed in babies who are born prematurely, especially in extremely premature infants. 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 and nervous system. They may have difficulty controlling body temperature and breathing, develop jaundice, abnormalities of their electrolytes and blood sugar and may be too immature to breast or bottle feed. They may require assistance in many areas for a time until they are more mature.

More information about specific medical conditions associated with prematurity can be found in the NICU Learning Hub.