I was reading this book last night called "From Conception to Birth" and it takes a scientific approach to pregnancy.

One thing that it said was that immediately after fertilization, the body recognizes the chemical change in the egg, and begins gently contracting muscles in the fallopian tubes and moving tiny hairs called cilia which push the egg down towards the uterus.

It also chronicles the sperms' journey up through the uterus to find the egg. There are an incredible number of hazards along the way, including white blood cells, which see the sperm as outside invaders and hunt them down and kill them. Out of hundreds of thousands of sperm, only a few dozen make it to the fallopian tubes. And then they must choose the left or right tube. Half of them will make the wrong decision. The neat thing about this process is that it ensures that only the strongest - and luckiest! - sperm survive.