Gestational age

Gestational age is the age of a fetus (or newborn infant) from the beginning of pregnancy. Although there is some controversy over the beginning of pregnancy, the definition most commonly used is from the woman's last menstrual period (LMP).

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 embryologic and fetal development usually occur at known gestational ages. This 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.