Gestational age is age of a fetus (or newborn infant) from presumed conception. A full-term human pregnancy is considered to be 38 weeks (266 days), though individuals will vary between 35 and 40 weeks. Note that these dates differ from calculations using the woman's last menstrual period (LMP): the average date for conception is fourteen days (two weeks) after the LMP date. A fetus born prior to the 35th week of gestation (the 37th week LMP) 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 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. Obstetrics