Fertilization

As the cells divide, the embryo continues its journey along the fallopian tube. About one week after conception, it reaches the uterus, which is prepared to receive it with the network of blood vessels and glands. By this time, the embryo has become a fluid-filled sphere of nearly 100 cells. Some of the cells become finger-like projections that anchor the embryo to the uterus to draw nourishment and oxygen and to rid the embryo of wastes. The anchoring cells secrete a hormone that will prevent the disintegration of the lining. It will be no menstrual period.

Three weeks after fertilization, though still smaller than a grain of rice, the embryo has a primitive heart. Through the following weeks, its tissues and organs will develop. In four weeks, it looks like this. Five weeks, about the size of a pea. Six weeks, seven weeks. At two months, it is called a fetus and is obviously human, though only the size of a gumdrop. When the embryo attaches to the uterus, a complex structure of vessels and cells forms, which is called the placenta. A network of the embryo's blood vessels closely intermingles next to the mother's blood supply.

While their blood doesn't actually mix, materials such as oxygen, nutrients and waste products can pass between mother and child. If their blood were to mix, the embryo would be rejected as the foreign organism that in fact it is. Don't forget, it shares only half of its genetic constitution with its mother. From the placenta emerges the umbilical cord, which leads to the abdomen of the fetus. When the cord is removed at birth, the belly button is created. During pregnancy, the placenta and umbilical cord serve as pathways for everything the fetus needs to grow.

By the third month, all the organs of the fetus are essentially in place and it enters a period of intense growth. During the fourth month, the fetus doubles in size. Its muscles twitch. By the fifth month, it is nearly 21 centimeters long, about 9 inches. It kicks occasionally. Its heart beats about 140 times per minute, twice the speed of an adult.