Gestational age is usually considered to be the age of an embryo or fetus (or newborn infant) from the first day of the woman's last menstrual period (LMP).  This standard system of counting the progression of pregnancy starts approximately two weeks before fertilization takes place; it does not in itself constitute the beginning of pregnancy. There is some controversy over the beginning of pregnancy, and alternative counting systems have been suggested.
The gestational age should not be confused with the fertilization age (also called conceptional age or developmental age) of an embryo or fetus; the fertilization age is always counted from fertilization, and the gestational age is usually greater by about two weeks.
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:
- The mother's knowledge of the date of sexual intercourse.
- 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.
- 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.
- 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 fertilization age of children conceived by in vitro fertilization is known to the hour.
- ↑ John Goldenring, ed. (November 2004). "MedlinePlus Medical Encyclopedia: Gestational age". A.D.A.M., Inc. Retrieved 2007-01-28. (distinguishing between "calendar gestational age" and "developmental gestational age").
- ↑ Anika Rahman, Laura Katzive and Stanley K. Henshaw. A Global Review of Laws on Induced Abortion, 1985-1997, International Family Planning Perspectives (Volume 24, Number 2, June 1998): there is "no uniform method of calculating gestational age."
- ↑ 3.0 3.1 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 2007-01-28.