Though frequently cited as a carrier for small numbers of sperm, and thus a potential cause of pregnancy, it is very unlikely that conception will occur from exposure to pre-ejaculate fluid. It has also been theorized that pre-ejaculate may push sperm from a prior ejaculation out, presumably if he has not urinated since his most recent ejaculation. These are among the reasons why withdrawal, or "pulling out", is not considered to be a reliable method of birth control. There have been no large-scale studies of sperm in pre-ejaculate, but some smaller-scale studies suggest that any sperm present may be ineffectual at causing pregnancy.
A more serious problem is those same studies have shown the presence of HIV, the virus responsible for the disease AIDS, in pre-ejaculate. Pre-ejaculate can also transmit other STDs.