The Human Body


Fetus, term applied to an animal embryo after a definite period has elapsed following conception. In human reproduction, for example, the period is eight weeks; for early embryonic development, see Embryology.

In the first half of the second month of gestation, the human embryo closely resembles that of other mammals, but in the latter part of the month the head becomes disproportionately large, principally because of development of the brain. The external genitalia also appear in the latter part of the second month. The extremities become more developed, and the fetus attains a length of about 3 cm (about 1.2 in).

By the end of the third month, centers of ossification appear in most of the bones, the fingers and toes become differentiated, and the external genitalia begin to show definite sex differentiation. After the fourth month the average fetus is almost 15 cm (almost 6 in) long and weighs about 113 g (about 4 oz). The sex of the fetus is easily identifiable. The face looks human, and movement is usually discernible. During the fifth and sixth months a downy covering called lanugo develops on the body, and the body becomes increasingly larger in proportion to the head. The fetus attains a length of about 30 cm (about 12 in) and weighs about 624 g (about 1 lb 6 oz).

During the seventh month the skin, which is red and wrinkled, is covered with a white substance called the vernix, or vernix caseosa, which protects the skin. The vernix is a mixture of epithelial cells, lanugo hairs, and secretions from the glands of the skin. By then the fetus is about 40 cm (about 15 in) and has attained a weight of more than 1 kg (more than 2 lb). The pupillary membrane disappears from the eyes. The body organs are sufficiently developed to sustain life outside the uterus; the more developed the fetus, the greater are its chances for extrauterine life. A fetus born at this period moves its limbs quite energetically and cries with a weak voice. After this period, during the eighth and ninth months, the fetus loses its wrinkled appearance due to the deposition of subcutaneous fat. The fingers and toes have well-developed nails.

Full term is reached at the end of the tenth lunar month of pregnancy. Most of the fetal hair has been shed, and the fetus is ready for birth, having attained a length of about 50 cm (about 20 in) and a weight of approximately 3 kg (approximately 7 lb). The vernix covers the entire surface of the body. When the infant is born before the full term and weighs less than 2.4 kg (5 lb 8 oz), it is considered premature.

Respiratory activity occurs in the fetus as early as the twelfth week of gestation and continues throughout its intrauterine life. The lungs do not function in any effective sense, however, because the fetus is enclosed in a sac that fills with a clear amniotic fluid early in the embryonic period. Oxygen and materials needed for nutrition are brought to the fetus from the placenta, a vascular organ which unites the fetus to the maternal uterus, by the umbilical vein. Conversely, the placenta is responsible for the conveyance of carbon dioxide and waste products from fetus to mother. The placenta has an increasing permeability as pregnancy advances. Metabolites, waste products of metabolism, gain access to the fetal circulation from the mother's blood by direct diffusion across the membranes and, in certain cases, by selective transfer of particles.