Remember that 50 percent of the fetus's genes are from the father and theoretically can carry information that could sabotage the relationship between mother and child. But the mother's immune system overlooks this fact and seeks to protect the child anyway. In a way, you can consider pregnancy an immuno-suppressed state; the volume of mom's immune system is turned down as it deals with this foreign, yet welcome, invader.
The most dangerous time for the newly created creature that is to become your child is prior to implantation in the uterine wall. If mom's immune system is not suppressed when the fertilized egg tries to land, it's bye-bye blastocyst. So exactly one day before implantation—six days after conception—the blastocyst produces a special enzyme that suppresses mom's killer T cells, preventing her from having an immune reaction against the baby-to-be's cells. Once the placenta is formed after implantation, it helps maintain the truce between these two potential adversaries.
Find out more about this book:YOU: Having a Baby: The Owner's Manual to a Happy and Healthy Pregnancy