Go Ask Alice at Columbia University says:
Sperm could be in pre-cum, but only after a recent ejaculation, after which some sperm may be left hanging around in the urethra. "Recent" means masturbating earlier and then having sex with a woman, or during the same sexual episode of the recent ejaculation. Urinating in between ejaculations flushes the urethra of stray sperm and makes the way clear for the sperm-less pre-ejaculate fluid. If sperm remains after a prior ejaculation, then it's possible that they can enter the vagina and make their way to meet an egg.
Sperm could be in pre-cum, but only after a recent ejaculation, after which some sperm may be left hanging around in the urethra. "Recent" means masturbating earlier and then having sex with a woman, or during the same sexual episode of the recent ejaculation. Urinating in between ejaculations flushes the urethra of stray sperm and makes the way clear for the sperm-less pre-ejaculate fluid. If sperm remains after a prior ejaculation, then it's possible that they can enter the vagina and make their way to meet an egg.