The Fetus in Utero Including Twins and the Placenta

Abstract

By definition, the fetus is a develo** mammal or other vivparous vertebrate. In humans, this is the unborn from the eighth week of pregnancy following fertilization (or 10 weeks after the onset of the last menstrual period), when in the embryo the major organs have been formed and continue their development until the moment of birth. At this time, the fetus is about 5 cm (2 in.) in length, weighs ~8 g, and the develo** head constitutes about one-half the total mass. The word fetus derives from Latin, bearing, bringing forth, or hatching of young. From weeks 11–17 of pregnancy the brain, heart, and other organs continue to develop, and subtleties appear in the various structures such as development of the genitalia and centers of ossification in bones. At about 16 weeks, a woman who has been pregnant previously will feel fetal movements, “quickening;” this may not occur, however, until about 20 weeks in those women who are nulliparous (having not delivered before). From weeks 18–27, development continues with the appearance of many structures such as the eyelashes and eyebrows, finger and toe nails, and a fine lanugo hair covering the body. From weeks 28–40 (term), nerve growth with myelinization continues, the amount of body fat increases, the cerebral electrocorticogram takes on cyclic activity, e.g., low voltage high frequency, during which metabolism of the brain increases, (and which is associated with rapid eye movements and other muscular activity), versus a high voltage low frequency sleep state. At this time, the fetus is capable of life independent of the mother’s womb; survival for the most immature has been enhanced greatly by advances in neonatal intensive care. At the end of the 36th weeks of gestation almost all infants born can survive independently of neonatal intensive care.