Gestation is the period of time between conception and birth. During this time, the embryo or fetus grows and develops inside the mothers womb. Gestational age is the term used during pregnancy to describe how far along the pregnancy is. It is measured in weeks, from the first day of the womans last menstrual cycle to the current date. A normal pregnancy can range from 38 to 42 weeks. Infants born before 37 weeks are considered premature, while infants born after 42 weeks are considered postmature. Gestational age can be determined before or after birth. Before birth, healthcare providers use ultrasound to measure the size of the babys head, abdomen, and thigh bone to determine gestational age. After birth, gestational age can be measured by looking at the babys weight, length, head circumference, vital signs, reflexes, muscle tone, posture, and the condition of the skin and hair. The average length of human gestation is 280 days, or 40 weeks, from the first day of the womans last menstrual period. However, only about 4% of women actually give birth on their estimated due date.