Definitions of Prematurity

This section introduces the terms medical professionals use to understand prematurity and associated conditions

What is Considered Premature?

A premature baby is defined as a baby born alive before they complete 37 weeks of pregnancy. There are three sub-categories of premature birth based on gestational age:

  • Extremely preterm (less than 28 weeks’ gestation)
  • Very preterm (28 to 32 weeks’ gestation)
  • Moderate to late preterm (32 to 37 weeks’ gestation)

What does ‘Low Birth Weight’ mean?

Often, the birth weight of a baby is a predictor of their health; the lower the weight, the greater the association with poor outcomes.

You may hear the medical staff use these terms:

  • Extremely low birth weight

A baby whose birth weight is less than 2 lb 3 oz (1000g)

  • Very low birth weight

A baby whose birth weight is less than 3 lb 5 oz (1500 g).

  • Low birth weight:

A baby whose birth weight is less than 5 lbs 8 oz (2500 g).

It is an important to note that not all low-birth weight babies are born prematurely.

What does ‘Small for Gestational Age’ mean?

You may hear the medical staff use the term ‘Small for Gestational Age’ (SGA).  This term is used to describe a baby who is smaller (lighter) than expected for the number of weeks of pregnancy.  More specifically, the term ‘Small for Gestational Age’ is used to define a baby whose birth weight is below the 10th percentile for that gestational age, i.e. of the babies born at the same gestational age, 90% of them are bigger.

You may also hear the term ‘Severe SGA’.  This means that the baby is considered to be less than the third centile, i.e. 97 % of all babies born at that gestational age are bigger than the Severe GSA baby.

Certain factors affect birth weight and are associated with normal variation in birth weight.  These factors include maternal height; maternal weight; parity; ethnic origin; and baby’s gender.

What is Intrauterine Growth Restriction?

Intrauterine Growth Restriction (IUGR) refers to a condition in which an unborn baby is unable to grow to the size it should have, determined by its genetics. IUGR may occur secondary to pregnancy complications and may lead to the birth of a ‘Small for Gestational Age’ baby.  IUGR is characterized by fetal growth less than normal, and less than the growth potential of the baby.

IUGR can be symmetrical or asymmetrical.  Symmetrical IUGR originates in early pregnancy and affects the whole body including weight, length, and head circumference.  Asymmetrical IUGR generally occurs later in pregnancy and causes a decrease in the baby’s weight without affecting length or head circumference.

For more information on IUGR see

Definitions of Prematurity

What is considered “premature”?

Why Has My Baby Come Early?

Reasons why your baby may have been born prematurely

What Preterm Babies Look Like

Premature babies generally appear differently to term babies