Babies swim in and drink their own pee for about 25 weeks.

Babies start to pee inside the amniotic sac around week eight, though urine production really picks up between weeks 13 and 16 (when kidney development is more complete).

They can start drinking this mix of pee and amniotic fluid around week 10 or 11, or when a layer of cells blocking their mouths — called the buccopharyngeal membrane — ruptures, allowing the baby to swallow. By week 20 most of the amniotic fluid is urine.

Cheers, kids.