You do realize women who don't have birth control protection get pregnant all the time. Most "pregnancies" spontaneously abort, usually before women know they were pregnant. Impregnation is far more common that most men realize. Even a lot of women don't realize how often they pass a fertilized ovum that failed to cling to the walls of the uterus for myriad reasons.

In addition, in most cases, when someone is planning and scheduling impregnation times, it's likely due to fertility issues. Those can be caused by stress or health problems. It can also be a matter of the day to day busyness of our lives that partners miss the critical window.