Can certain positions make the possibility of pregnancy less likely?

Dr. Sawyer: Standing up, fully clothed at least 15 yards from an erect penis is about the only sure position to prevent pregnancy! Gravity might play a minor role in decreasing the likelihood of pregnancy, so I guess, “woman on top” might fractionally decrease the odds, but not by nearly enough for anyone to even consider this as a means of organic contraception. Some might argue that the missionary position (man on top) provides the shallowest penile penetration, thus making the sperm travel further in a hostile environment that tends to kill them, but given that a typical male’s ejaculate contains approximately 350 million sperm, you can begin to see why sexual positions aren’t so relevant.