''hCG and Anti-hCG Antibodies
In nature the hCG hormone alerts the woman's body that she is pregnant and causes the release of other hormones to prepare the uterine lining for the implantation of the fertilized egg. The rapid rise in hCG levels after conception makes it an excellent marker for confirmation of pregnancy: when a woman takes a pregnancy test she is not tested for the pregnancy itself, but for the elevated presence of hCG.
However, when introduced into the body coupled with a tetanus toxoid carrier, antibodies will be formed not only against tetanus but also against hCG. In this case the body fails to recognize hCG as a friend and will produce anti-hCG antibodies. The antibodies will attack subsequent pregnancies by killing the hCG which naturally sustains a pregnancy; when a woman has sufficient anti-hCG antibodies in her system, she is rendered incapable of maintaining a pregnancy.''