You can get pregnant any which way you have vaginal sex. Just think, out of ALL of the unplanned pregnancies happening everyday, how many women do you think have their legs up the wall? That said, missionary (him on top) and doggie-style positions (him behind) do allow for deeper penetration that could bring the sperm closer to the cervix. Though you don’t need to stay lying down afterwards, it can’t hurt either, right? Placing a pillow under your back and thus tipping your pelvic area up could also encourage those little swimmers to get where they’re going.
