Gestation is the period of time between conception and birth during which the fetus grows and develops inside the mother's womb.

Gestational age is the time measured from the first day of the woman's last menstrual cycle to the current date. It 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. The gestational maturity rating is measured by the Ballard scale or Dubowitz exam.

Gestational age can be determined before or when the baby is born.

Prior to birth, growth is determined with ultrasound by measuring the diameter of the head and comparing the head circumference and the abdominal circumference.

Following birth, assessing an infant's weight, length, head circumference, condition of skin and hair, reflexes, muscle tone, posture, and vital signs can provide a 'evelopmental gestational age. This 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 full-term infant than a premature infant.

Determination of gestational age is important because it provides valuable information regarding expected or potential problems and directly affects the medical treatment plan for the baby.