The fundamental problem in achieving a successful pregnancy is the woman's bodily defences seeing the father's part of the embryo as a foreign body.

Using the example of organ transplants, only when the donor's tissue is similar to the recepient's, can organ rejection be avoided. However, mechanisms do exist to protect the pregnancy from an attack by the woman's immune system.

Antibodies "protect" the pregnancy by blocking some of the mother's immune cells. The immune system would immediately recognise the embryo as a foreign body but is inhibited from doing so by these antibodies .. It is presumed that some similarity between the partners prevents the woman's immune system from developing protective antibodies.

One theory for treatment is "active immunisation", when the woman's immune system is prompted to produce "protective antibodies" by her white blood cells.

Another theory is "passive immunisation"; it was found that when immunoglobulin infusions were applied during organ transplants, rejections were rare. The same method was used to prevent the rejection of embryos by the woman's body.