Published December 12, 2007 by
While your friends and family members are eagerly brainstorming baby gift ideas, your body is going to be busy making some drastic changes! Conception occurs two weeks after the women's last menstrual period. The egg and the sperm fuse to produce one cell. In the first three months, or trimester, the embryo takes shape and all the organs are formed. In the last six months, the fetus grows and matures.
In the first weeks after conception, the single cell rapidly divides into many cells. A hollow ball of cells is formed and becomes attached to the womb. Some of the cells will become the placenta; the rest will become the embryo. The latter group of cells develops into a four-layered disc. Each layer will be converted into different areas of the body. The outer layer of ectoderm, for example will develop into skin, hair, nails and the nervous system. The inner layer, or endoderm, will develop into the intestines and lungs. The middle layers will develop into the heart, bones, and muscles.
By three weeks after fertilization, or about one week after the first period is missed, the embryo already is one-tenth of an inch long and has an oval shape. In the next few weeks, it becomes more curved in shape and a head and tail are discernible. The beginning of the spinal cord and brain take shape. A tubular heart begins to form. Tiny eyes can be seen. Arms and legs begin to bud.
By the fourth week after fertilization, traces of all the organs of the body are present. Bulges that become the ears and nose appear. The gut is formed from blind pouches within the embryo; these push forward, creating an opening in the head that will become the mouth. A crude face begins to take shape. At this point, the embryo is only one quarter of an inch long.
The embryo is called a fetus at the seventh or eighth week. It has grown to be the length of one inch; the head is disproportionately large because of the size of the developing brain, while the abdomen seems large because of the growing liver. Fingers and toes appear. The rudiments of all the hormone- producing glands-the pituitary, thyroid, and the adrenal glands-are present. Amazingly, the tiny heart begins to beat.
By the end of the third month, the fetus is two to three inches long and weighs less than an ounce. Nails form on the fingers and toes. The bones begin to calcify. The male or female sex organs begin to develop. The tooth buds form in the mouth. The fetus begins to make breathing movements and starts to swallow amniotic fluid. The muscles in the intestines begin to contract and relax, as if digesting food. Skeletal muscles begin to work as well, so the fetus can move to response to local pressure.
Although the organs are present by the end of the first trimester, the fetus is not yet able to live outside the mother's body. The second trimester is devoted to primarily to maturation of the organs. By the fourth month, the fetus moves spontaneously but is too small for the mother to feel. The fetus is four to five inches long and weighs three ounces.
By the fifth month, however, the baby is six inches long and weighs one half pound, and is strong enough to make his presence felt. The mother's perception of the baby's movement is known as quickening. In the fifth or sixth month, the body becomes covered in fine hair, or lanugo and coarse hair appears on the head.
The baby is fully developed by the beginning of the third trimester. The last three months, therefore, are devoted to growth. The baby is about ten inches long and weighs one to two pounds by the seventh month. The skin is red, wrinkled and thin. It becomes covered in vernix, a thick, whit, sticky material composed of skin cells, ` and oily skin secretions. If the baby were born at this time, he would have a fifty percent chance of survival, provided he received appropriate medical care. Babies born this early can respond to taste, light, and sound.
If the baby is born in the eighth month of gestation, his chance of survival increases to ninety percent. By this time, he is ten to twelve inches long and weighs three to four pounds.
The final preparations for independent existence occur during the ninth month. Surfactant, a substance that lines the lungs and allows them to expand easily, develops. Fat is stored, and its deposition under the skin smooths out the wrinkles. Much of the lanugo disappears.
By the final month of pregnancy, the fetus is usually fourteen to sixteen inches long and weighs seven to eight pounds. He is large and strong enough for the next step-birth and independent life. That one cell has come a long way, from embryo, to fetus, to newborn baby.