Gestational age can be determine before or after birth.
Before birth, your doctor will use ultrasound to measure the size of the baby's head, abdomen, and thigh bone. This helps determine how well the baby is growing in the womb.
After birth, gestational age can be measured by looking at the baby's weight, length, head circumference, vital signs, reflexes, muscle tone, posture, and condition of the skin and hair. The baby's developmental gestational age may not be the same as the calendar age. For example, an infant born at 36 weeks may actually have a developmental gestational age of 38 weeks. Such a baby may be more like a full-term infant than a preemie.
Determination of gestational age is important. It gives your doctor key information and directly affects the medical treatment plan for the baby.