If IMPLANTATION occurs and the right environment surrounds the embryo it continues to develop into a fully fledged fetus ready for delivery forty weeks later.
If any part of the processes goes wrong, either fertilization or implantation, the thick endometrium gets a signal to shed and menses occurs four days later. And the cycle begins again!
This is just a simple explanation. It sound treacherous, exciting, exhausting and remarkable- and it is truly amazing!