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[L. fetus, bearing young, fetus, offspring]

1. An unborn animal in its later stages of development.
2. The unborn human from the beginning of the ninth week, i.e., the third month, of gestation until birth. (Before 9 weeks, the developing zygote is called an embryo.) During the fetal period, the organ systems mature and grow. At 9 weeks, a human fetus weighs about 14 g (1/2 oz); in the subsequent approx. 29 weeks, the fetus becomes 250 times heavier, so a typical newborn weighs about 3500 g (7 3/4 lb). A human fetus gets all its nutrition through the placenta, which increases in size with the fetus.

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