Gestational age
Gestational age is age of a fetus (or newborn infant) from presumed conception. A full-term human pregnancy is 40 weeks, or 280 days. 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.
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