This involves cell division, cell migration, and the differentiation of cells into the many types found in the baby. The tranquilizer, thalidomide, taken by many pregnant European women between 1954 and 1962, turned out to be a potent teratogen and was responsible for the birth of several thousand deformed babies. By 3 months, all the systems of the baby have been formed, at least in a rudimentary way. From then on, development of the fetus , as it is now called, is primarily a matter of growth and minor structural modifications. The fetus is less susceptible to teratogens than is the embryo.
Pregnancy involves a complex interplay of hormones .
These are described in a separate page. The placenta is an allograft One of the greatest unsolved mysteries in immunology is how the placenta survives for 9 months without being rejected by the mother's immune system. While X-chromosome inactivation is random in the cells of the fetus, it is NOT random in the cells of the trophoblast. But this does not solve our problem because the genes for all the major histocompatibility antigens are located on chromosome 6, which is not inactivated. Thus the placenta is immunologically as foreign to the mother as a kidney transplant would be.