4.2.1 DEVELOPMENT OF EMBRYO AND FETUS
As the fertilized egg develops into the baby we see at birth, it goes through three main stages of growth: 1. the stage of the zygote (before implantation), 2. the stage of the embryo (early in the pregnancy), and 3. the stage of the fetus (later in the pregnancy). Since the first of these stages has already been discussed (see "Conception"), we now turn to a description of how the new organism grows inside the uterus.
The word embryo (Greek: swelling within) refers to the growing organism from the second to the eighth week of its life. During this time, it develops from a tiny cell cluster into a little growth of about 1 inch in length. As this development proceeds, the placenta, a special organ of interchange, begins to grow between the embryo and the uterus. The embryo is connected to the placenta by the umbilical cord. (Soon after the birth of the baby, its umbilical cord is still connected to the placenta which is then expelled from the uterus. For this reason, the placenta is also called the afterbirth.) The placenta acts as a filter and as a barrier. It allows the embryo (and later the fetus) to absorb food and oxygen from the woman's blood and to eliminate carbon dioxide and other waste from its own blood in return. At the same time, however, the two blood systems remain completely separate.
During the first month of its life, the human embryo looks like that of any other higher animal, such as a cat, dog, or pig, for example. Then, during the second month, it slowly assumes human features. It starts to develop a recognizable face, as well as arms, legs, fingers, and toes. Between its legs, the primitive beginnings of sexual organs become discernible, although they are still undifferentiated at this point (i.e., they are the same for both male and female). When the entire growing organism finally becomes clearly identifiable as human, it leaves the stage of the embryo and enters that of the fetus.
The word fetus (Latin: offspring) is used to describe the growing organism from the beginning of the third month of its life to the moment of birth. During this time, it develops from a small growth of slightly over an inch weighing only a fraction of an ounce into a baby of about twenty inches in length weighing approximately seven pounds. In the first weeks of this development, the male-female sex differentiation becomes apparent in the internal sex organs. A little later, the external sex organs develop their characteristic structure. Sometime around the fifth month, the fetal movements become strong enough to be felt by the expectant mother. This so-called quickening was formerly believed to be the moment when life entered the new body.
Throughout its growth, the fetus is well protected from injuries as it floats almost weightlessly inside a fluid-filled sac called the amniotic sac.
DEVELOPMENT OF EMBRYO
1. 4 weeks 2. 5 weeks 3. 6 weeks 4. 7 weeks 5. 8 weeks
At the end of the sixth month, the fetus measures about six inches in length and weighs about one and a half pounds. At this time, the centers of the brain which control breathing begin to develop. It is not entirely impossible (although very unlikely) that such a fetus could actually survive a premature birth. However, the probability of brain damage because of still ineffective breathing is great. The last months of fetal development bring further refinements, such as the temperature control mechanism in the brain and a protective layer of fat under the skin. In the case of a male fetus, the testicles descend into the scrotum. If this descent should fail to occur, corrective measures have to be taken sometime after birth. Otherwise, sterility will result. (For details, see "Sexual Malformations.")
During the final weeks before birth, the fetus not only grows rapidly in size, but also gains much of its weight. The birth of a fetus weighing less than five pounds is premature.