Sheena Lewis, PhD, Professor and Director of the Reproductive Medicine Research Group at the Queen's University, directed a study which was covered in a Mar. 31, 2004 article by BBC News:

The study … examined the direct effects on sperm function of THC, the active ingredient in cannabis. The group found that THC made sperm less likely to reach the egg to fertilise it.

They also discovered that the presence of cannabis impaired another crucial function of sperm – the ability to digest the egg’s protective coat with enzymes to aid its penetration.

Mar. 31, 2004