Most babies hold their poop until birth.

Called meconium, a baby's first stool is made of all the skin, hair, bile, proteins, white blood cells, and other stuff that floats in the amniotic fluid — because a baby drinks it all for 20 to 25 weeks.

Meconium starts forming as soon as a baby opens its mouth and begins swallowing amniotic fluid, around week 11, but meconium production really picks up by week 19 or 20 as a fetus matures.

It typically comes out after birth as a greenish-black, tarry, and odorless mess. Or at least this was the case for my baby; other colors (like white) can indicate a serious medical condition.

And while 13% of babies do poop in the womb, too much meconium in the amniotic fluid can block a baby's airways before birth, leading to an oxygen-deprived state called fetal distress.

Sources: Columbia University, HealthyChildren.org, American Pregnancy Association, Clinics in Perinatology