The Haploid Affair(1986)

An ejaculate of sperm swim through a uterus. At the branches of the Fallopian tubes, half go one way and half the other. Down one tube toward the sperm bounces a female head, singing, happily on her way. Four sperm zero in; she panics but can't avoid them. Pow! they hit her full force and disappear into her. The decision she makes and what she does next change the course of her life in a major way.