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Gestational age is the age of an embryo or fetus (or newborn infant) from the first day of the woman's last menstrual period (LMP).^[1] It is a standard system of counting the progression of pregnancy and starts approximately two weeks before when fertilization takes place, it does not in itself constitute the beginning of pregnancy.^[2] There is some controversy over the beginning of pregnancy, and alternative counting systems have been suggested.^[2]

The gestational age should not be confused with the fertilization age of an embryo or fetus; the gestational age is always greater.

A full-term human pregnancy is considered to be 40 weeks (280 days), though pregnancy lengths between 38 and 42 weeks are considered normal. A fetus born prior to the 37th week of gestation is considered premature and faces increased risk of morbidity and mortality.

The events of prenatal development usually occur at known gestational ages. The gestational timing of a toxin exposure or infection can be used to predict the potential consequences to the fetus.

Calculations of gestational age from LMP are sometimes incorrect due to normal varation from the average ovulation date. The gestational age of an individual infant can be more accurately estimated from:

1. The mother's knowledge of the date of sexual intercourse.
2. The mother's knowledge of fertility signs related to ovulation. The needed observations are normally made by mothers who use fertility awareness methods to get pregnant.
3. Examination of the newborn infant. In the twentieth century, doctors (especially pediatricians) were trained to recognize the physical changes occurring to the fetus in the latter half of pregnancy so that a maturational age could be estimated.
4. An obstetric ultrasound ("dating scan", in the UK routinely offered around 12 weeks) during the pregnancy, whereby sizes of certain fetal body parts are measured.

The gestational age of children conceived by in vitro fertilization is known to the hour.

• pregnancy
• maternity

1. ^ John Goldenring:MedlinePlus Medical Encyclopedia: Gestational age. A.D.A.M., Inc. (November 2004). Retrieved on January 28, 2007.
2. ^ ^{a b} Engle W, American Academy of Pediatrics Committee on Fetus and Newborn (2004). "Age terminology during the perinatal period". Pediatrics 114 (5): 1362-4. PMID 15520122. Retrieved on 2007-01-28.