Gestational age is age of a fetus (or newborn infant) from presumed conception. A full-term human pregnancy is 40 weeks, or 280 days. Fetus at eight weeks A fetus (alternatively foetus or fœtus) is an embryo in later stages of development, from the third month of pregnancy until birth in humans. ... The term conception can refer to more than one meaning: Concept Fertilisation This is a disambiguation page — a navigational aid which lists other pages that might otherwise share the same title. ... A pregnant woman Pregnancy is the process by which a mammalian female carries a live offspring from conception until it develops to the point where the offspring is capable of living outside the womb. ...

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.

Traditionally, a gestational age of an individual infant has been estimated from

• (1) mother's knowledge of date of conception (assuming there was only a brief period of sexual contacts), or
• (2) calculating from her last missed menstrual period (assuming that ovulation and conception would have occurred about 2 weeks after the last menses). 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
• (3) a gestational age could be guessed by examination of the newborn infant. In recent decades, the common use of ultrasound during the pregnancy has begun to provide still another
• (4) method of estimating gestational age (based on the measured sizes of certain fetal body parts). Finally
• (5), the gestational age of children conceived by in vitro fertilization is known to the hour.