manieedomingue,
Pre-ejaculate does contain sperm, but not very much. Though it only takes one sperm to actually fertilize the egg, it does take a lot more than that (millions) to actually break down the corona around the egg.
So, the chances of getting pregnant from pre-ejaculate itself is pretty small. However, the withdrawal method is not a very good one for birth control. It is not really possible to tell how much fluid is actually transferred.
Again, there is a small amount of sperm in the pre-ejaculate, exactly how much, you can't really tell.
Good luck.