From Dr. Trost: I'm not sure if there is a definitive answer to this question. I did a quick literature search to see if anything had ever been published on this, and I was not able to find anything specific. It would seem to make empiric sense to me that the optimal position for the female would be laying on her back as this would increase the amount of time that sperm are present in the correct location near the cervix. It is interesting, however, that sperm are able to travel to the location of the egg within a few minutes of ejaculation. This would argue that the actual position maybe less important than one would think.