During heterosexual anal sex, splash conception occurs when semen containing sperm migrates from the anus, across the perineum, to the labia, and subsequently into the vagina, causing the fertilization of an ovum followed by conception or pregnancy. This migration does not occur by virtue of the spermatozoa's tail propulsion, but rather due to gravity or other kinetic forces (such as that of a penis pulling out of an anus and dragging semen left on the tip, or being expelled from the anus due to said penis's recent ejaculation, across the perineum and onto the labia.
