What it is: Transitional stool When: Days 1- 5 (or so) What you need to know: Over the first few days of life, you'll notice your baby's poop changing from the classic, thick, tarry meconium to something lighter colored and not quite so sticky. His poop may literally look different from one diaper change to the next -- and that's okay! It takes time for your baby to empty out his pre-natal waste. In the meantime, he’s starting to take in fluid -- colostrum and breastmilk, or formula. New food in = new poo out. How quickly your baby passes through this stage is strictly a function of how much he takes in. If he nurses a lot and your milk comes in on day two, the transitional poop stage may only last a day. If it takes him longer to ramp up his intake, the transitional stage may last until almost the end of the first week.