Gestational age refers to how many weeks the fetus has been growing inside the mother’s womb. It is based on the date of the mother’s last menstrual period. An ultrasound during the first 3 months of pregnancy measuring the head, abdomen, and thigh bone also helps to determine gestational age.
Most babies are born after 38 to 42 weeks of gestation. Right after birth, your doctor checked your child’s weight and length, and compared it with the weight and length expected for his or her gestational age. Most babies have a birth length and weight very close to what is expected, but some are larger or heavier than expected, and some are smaller or lighter.
Your doctor may also have looked at other things, such as your baby’s head size, the condition of skin and hair, reflexes, muscle tone, posture, and other signs to determine your baby’s developmental gestational age. This second number describes the age that your baby actually looks and acts. For example, although your baby may have been born after 39 weeks, he or she may look and act more like a baby born after only 35 weeks.
Both of these gestational ages are important because together they provide valuable information that can directly affect the medical treatment plan for your baby.
If you are told that your child is small for gestational age (SGA), it means that his or her birth weight and/or length was below the 3rd percentile. This means that your child was smaller than 97% of all other babies at that gestational age.
About 100,000 children born in the United States each year are shorter and lighter than normal, whether or not they are premature or full-term infants. Most children born SGA catch up, but those with no catch-up growth by 2-4 years of age may need more attention from an endocrinologist, a doctor who specializes in endocrine care.
Most of the time, we don’t know why a child is born SGA. It just happens. When we can locate a cause, the 3 most common reasons are issues with the fetus, including a multiple birth (twins, triplets, etc) or genetic defect; problems with the placenta that do not let the fetus get enough blood; or mother’s health issues such as heart disease or malnutrition, or drug, alcohol, or cigarette abuse.
Children born SGA may have a normal amount of growth hormone, or they may have less growth hormone than the average child.