However, those millions of little squiggly swimmers do encounter some major obstacles along the way to fertilise that little egg. It's a long and hazardous journey for the sperm to meet up with that one little egg. The vagina itself is a hostile, acid environment that's tough on a sperm, not to mention the cervical mucus-cervix which forms a pretty effective road block all by itself, although it does thin out on your cycle's most fertile days. Then the sperm must continue on through the uterus and to the fallopian tube. And if the sperm and egg do happen to meet, the sperm must burrow into the egg's outer layer. Once the genetic material combines, you have an embryo. The embryo travels the remainder of the way down into the uterus and implants itself in the uterine wall and voilà! You're pregnant.
Your doctor calculates your due date by counting 40 weeks from the start of your last menstrual period. Why the two bonus weeks of pregnancy? Your doctor can't be certain of the date when you last ovulated, but you can be sure of the last date your cycle started, so the count begins with that date.