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Gestational age


Gestation is the period of time between conception and birth. During this time, the baby grows and develops inside the mother's womb.

Gestational age is the common term used during pregnancy to describe how far along the pregnancy is. It is measured in weeks, from the first day of the woman's 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. Infants born after 42 weeks are considered postmature.

Alternative Names

Fetal age; Gestation


Gestational age can be determined 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 the 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. This baby may be more like a full-term infant than a baby born prematurely.

Determining a baby’s gestational age is important. It gives your doctor information that help set the baby’s medical treatment plan.


Benson CB, Doubilet PM. Fetal measurements: normal and abnormal fetal growth. In: Rumack CM, Wilson SR, Charboneau JM, Levine DL. Diagnostic Ultrasound. 4th ed. Philadelphia, PA: Elsevier Mosby; 2010:chap 42.

Review Date:12/4/2013
Reviewed By:Neil K. Kaneshiro, MD, MHA, Clinical Assistant Professor of Pediatrics, University of Washington School of Medicine, Seattle, WA. Also reviewed by David Zieve, MD, MHA, Bethanne Black, and the A.D.A.M. Editorial team.

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