Here is what the Bedsider has to say about this subject: 

"First off, we have very limited scientific information about pre-cum so there can be confusion about it even among experts. Three small studies from years ago found no sperm in pre-cum, but there were only 43 guys in all of these studies combined. Some of the men in the studies had health problems, and it appears that the pre-cum samples they provided were not analyzed immediately so it may have been it difficult to tell if their sperm were swimming normally.

A more recent study had 27 healthy guys, some of whom gave multiple samples of pre-cum. The researchers analyzed the samples immediately and found that about a third contained live, swimming sperm. Popular advice says that sperm found in pre-cum may come from a previous ejaculation and can be flushed out when a guy pees, but the guys in this study who had peed after their last ejaculation still had sperm in their pre-cum. The bottom line is that this study can’t tell us whether pre-cum can cause a pregnancy, but it does tell us that it might. It also suggests that pulling out may work better for some guys than others—but unless you have a pre-cum sample and awesome microscope skills, you can’t tell which group a guy is in. This may be part of why even withdrawal rock stars sometimes have accidental pregnancies."

What do you think?