Prenatal Development

Almost right after conception, cell division begins. While the zygote is splitting and new cells are created, it moves through the mother's fallopian tube toward the uterus, the place it will call home and where it will receive nourishment for the rest of its prenatal days. As with any other "egged" living thing, the yolk of the ovum provides all necessary nourishment.

By the time the cell cluster arrives at the uterus on the first stop of this journey, the process known as differentiation is just beginning. Here cells separate into groups according to their future roles. At this point the blastocyst (fertilized ovum) is a hollow ball of cells. Part of these cells will begin to form four membranes that help protect the growing organism. These membranes will eventually become the yolk sac, the allantois (which later becomes part of the circulatory system), the amnion (which soon forms the amniotic sac, the fetus's bubble-like home), and the chorion, which later becomes the placenta.

The blastocyst literally "floats" for some time in the uterus, and by the sixth day after conception it finds its home by implanting itself in the uterine wall. This is a critical point in gestation, because if the blastocyst does not implant itself properly and at the right time and in the right place, the cell mass will die before it can reach the embryo stage. If all goes well, the blastocyst will be firmly embedded about two weeks after conception.