The obstetricians and gynecologists, because the vast majority of fertilizations do not result in a fetus, and the mother doesn't show any signs of it, the mother doesn't know, there's no change in her body, you can't test anything about the body, they consider implantation, where the early embryo sits into the uterine wall and at that point there's hormonal signals going-- bouncing back and forth between the fetus and the maternal one. 