Gestation is the period between conception and birth of a baby, during which the fetus grows and develops inside the mother's uterus.

Gestational age is the time measured from the first day of the woman's last menstrual cycle to the current date and is measured in weeks. A pregnancy of normal gestation is approximately 40 weeks, with a normal range of 38 to 42 weeks.

Infants born before 37 weeks are considered premature. Infants born after 42 weeks are considered postmature.

Strictly speaking, gestational age of the fetus or infant is a measurement of time in utero (inside of the uterus). Gestational age can be determined prior to birth or at birth.

Prior to birth, intrauterine fetal growth can be determined using ultrasonography by taking serial determinations of the biparietal diameter of the head and the head-to- abdomen circumference ratios.

Following birth, assessing an infant's weight, length, head circumference, condition of skin, hair, reflexes, muscle tone, posture, and vital signs can provide a 'relative' or 'developmental' gestational age. The 'developmental' gestational age may not match the calendar gestational age.

For example, an infant born with a gestational age of 36 weeks may actually have a developmental gestational age of 38 weeks, and therefore behave more like a term infant than a premature infant.

Determination of gestational age is an important factor in planning appropriate care for the fetus or infant. It provides important information regarding expected or potential problems and directly impacts the medical treatment plan for the baby.