A baby’s weight at birth (birth weight) can be a good indicator of a baby’s health. Birth weight is associated with a baby’s gestational age at birth, so if a baby is born early, he/she will generally be smaller than babies born later.
The ‘gestation’ of a baby means how long the baby has spent in the womb before birth or how far along the pregnancy is. Gestational age is measured in weeks and days from the first day of the mother’s last menstrual cycle (period). The length of a pregnancy varies but a normal pregnancy (also known as ‘full term’ or ‘term’) ranges between 38-42 weeks. Babies born before 37 weeks are considered ‘premature’.
The ‘average’ weight of a baby also depends on their sex, so this is taken into account as well as the gestational age. A baby in the normal range for their sex and gestation are said to be appropriate for gestational age (AGA). Babies weighing less than this are considered small for gestational age (SGA) and infants weighing more, are considered large for gestational age (LGA).
SGA can be defined as babies with a birth weight (and/or birth length) below -2 SDS (standard deviations below the average for gestation and sex) or below the 2.3rd percentile.
LGA can be defined as babies with a birth weight (and/or birth length) above +2 SDS (standard deviations above the average for gestation and sex) or above the 97.7th percentile.
Most babies (about 85%) that are born SGA will have catch-up growth in the first years of life (usually by the age of 2-3 years) and will not be short in childhood or as an adult. The 15% of babies who do not catch-up (height below –2 SDS after 2 or 3 years) are termed SGA with failure to catch-up, or sometimes just SGA. In most cases there is no known cause for this and these children are at high risk of remaining short in later life. Growth hormone therapy may help in some cases.