Definition

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 gestation.

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

Alternative Names

Fetal age - gestational age; Gestation; Neonatal gestational age; Newborn gestational age

Information

Gestational age can be determined before or after birth.

• Before birth, your health care provider will use ultrasound to measure the size of the baby's head, abdomen, and thigh bone. This provides a view on 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.

If the baby's gestational age findings after birth match the calendar age, the baby is said to be appropriate for gestational age (AGA). AGA babies have lower rates of problems and death than babies that are small or large for their gestational age.

The weight for full-term infants that are born AGA will most often be between 2,500 grams (about 5.5 pounds [lbs] or 2.5 kilograms [kg]) and 4,000 grams (about 8.8 lbs or 4 kg).

• Infants weighing less are considered small for gestational age (SGA).
• Infants weighing more are considered large for gestational age (LGA).

References

Nock ML, Olicker AL. Tables of normal values. In: Martin RJ, Fanaroff AA, eds. Fanaroff and Martin's Neonatal-Perinatal Medicine. 12th ed. Philadelphia, PA: Elsevier; 2025:Appendix B, 2118-2166.

Putra M, Hobbins JC. Obstetric ultrasound: dating, imaging, and growth. In: Landon MB, Galan HL, Jauniaux ERM, et al, eds. Gabbe's Obstetrics: Normal and Problem Pregnancies. 9th ed. Philadelphia, PA: Elsevier; 2025:chap 9.

Simmons RA. Abnormalities of fetal growth. In: Gleason CA, Sawyer T, eds. Avery's Diseases of the Newborn. 11th ed. Philadelphia, PA: Elsevier; 2024:chap 4.