She says "the rationale is that this position allows for deep penetration so the man's sperm can be ejaculated as close to the cervix (the opening of the womb) as possible, "this gives the sperm cells a flying start on their long journey." Dr Glenville's claims that the closer the sperm cells are to the egg boosts their chances of reaching it, though there is no scientific proof backing it.While offering explanation, she stated that any position that goes against gravity will discourage the sperm's journey.
She also pointed out another position which makes the man to enter the woman from behind while she knees in front of him with her bottom raised higher than her head. But other experts, including biologists Robin Baker and Mark Bellis, believes the woman's ability to orgasm can make a difference when it comes to conceiving.
source: The Mirror