Newborn poop is different from older infants and certainly different than ours.
All newborn poop starts off as the black, tarry material called meconium. Meconium is what’s in the large intestine after waiting all during pregnancy for the opportunity to poop. It is always dark, and always tarry and sticky. Once your baby is born, the colon begins its lifelong cycle of passing poop along and out of the body. About 10% of babies will actually start pooping the day of birth prior to delivery, but everyone starts pooping after birth.
It takes 1-3 days for all the meconium to pass, and then a new stage begins, either with breast feeding or formula feeding. Breast fed infants have a unique stool. This is because breast milk is a unique food. Breast feeding is like a blood transfusion. Start with breast milk, it is the product of the breast filtering the mother’s blood. Blood flows through the breast, and the filtration process creates breast milk. This means the milk contains only human proteins and material. Once swallowed, the baby’s intestine can absorb all of the breast milk directly into their bloodstream. The gut has little to do, as there is no decision about what to absorb, or digesting the material, it is ready to go directly into the baby’s bloodstream. This leaves essentially no waste, and so breast fed infants’ stool is mostly simply left over intestinal juices, the famous yellow, mustard seedy stool that is very watery.
So, if you nurse, expect there to be few if any formed stools, but rather a steady flow of watery, yellow stool with seedy material. Sometimes the colors in stool can vary from yellow to green and back, this is normal too.
Formula is like all other foods, it is not human material, so the gut must do two things with such food meals. First, it must decide what to absorb and what to discard. And then, alter the nutrients to be brought into the body so that they can enter the body and be used properly, aka, digestion. This takes more time than the transfusion-like process of nursing, and it leaves waste, creating a more solid poop. So, formula fed infants will have less frequent and more solid stools.
One last point has to do with infants’ level of effort to poop. All newborns start with an open anus, stool almost drops out as it arrives at the anus. But by 4-6 weeks of age, the anus shifts to a default closed position. Now the baby has to learn to relax the anus to poop. That takes some learning, and many, if not most, young infants understandably hesitate and close their anus as poop approaches to leave. This leaves the infant pushing against their closed anus, making for much crying and grunting. This is not constipation. Constipation is when the poop is too big and hard to pass comfortably. So if your 4-6 week old infant struggles to poop, but when the poop comes out it’s soft, it’s not constipation, it’s he or she learning to use her anus, a problem no parent can fix, you must let your baby learn.