fetus term used to describe the unborn offspring in the uterus of vertebrate animals after the embryonic stage (see embryo).
If the fetus is expelled before 36 weeks of gestation are completed, it often can survive outside the womb, but artificial assistance, such as intravenous feedings and strict maintainance of the ambient temperature, may be needed during the remainder of its normal developmental period.
The circulatory system of a humanfetus works differently from that of born humans, mainly because the lungs are not in use: the fetus obtains oxygen and nutrients from the woman through the placenta and the umbilical cord.
In the fetus, there is a special connection between the pulmonary artery and the aorta, called the ductus arteriosus, which directs most of this blood away from the lungs (which aren't being used for respiration at this point as the fetus is suspended in amniotic fluid).
The anatomy of the area surrounding a fetus, however, is different in litter-bearing animals compared to humans: each fetus is surrounded by placentaltissue and is lodged along one of two long uteri instead of the single uterus found in a human female.
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