1. After Ovulation, Fertilization Must Occur In 12-24 Hours
While sperm can live in a woman's body for 1-5 days, the window for fertilization is actually quite small after the egg is released. Couple this with the fact that many women don't ovulate in the middle of their cycles and timing sex to make a baby can be tricky without fertility awareness methods.

An interesting side note, the cervical fluid in a women's body helps to nourish the sperm while it waits for an egg to be released.