Early poop is meconium: a black or dark-green, thick, sticky, tar-like substance. It’s made up of intestinal cells, lanugo (the downy hair some babies are born with), mucus, amniotic fluid, bile, and water. Although it doesn’t smell bad, it’s much harder to clean up than regular poop (you can apply a layer of petroleum jelly to your baby’s skin to act as a barrier).
By the end of the first week, baby poop is made up of bile from the liver, which is added to food when it leaves the stomach, along with bacteria and undigested components of milk. If you’re breastfeeding, your baby’s poop will be bright yellow, watery, sometimes seedy, and lightly yeasty-smelling. Formula-fed babies can produce a browner, thicker poop, but it may still have a yeasty scent. The aroma comes from the fermentation of sugars in milk, similar to the process that makes bread rise
When your baby starts eating solid food, a range of healthy bacteria populates his intestines, producing browner, smellier poop. Healthy poop will range from mushy to firm, but hard pellets might indicate that your baby is constipated or dehydrated, so consult your doctor if you see two or three of this type of diaper in a row. Don’t be alarmed if you see food in his diaper as his digestive system learns to cope with solids. Undigested vegetables as well as iron-rich foods or supplements can also make his poop appear dark green.
Bright-green poop is usually a sign that food has moved rapidly through your baby’s system, perhaps because of a mild tummy bug. If your baby has smelly gas along with greenish poop, it could be the result of her drinking too quickly. Discuss strategies for slowing it down with your pediatrician; once that’s taken care of, her poop should return to normal. If it doesn’t, you may want to consult with the doctor about a possible food intolerance. If you see other colors in poop, like red, white, black (after the meconium stage), or pale yellow, also see your doctor to rule out various causes. Silver, maroon, or raspberry gel–like poop needs urgent medical attention.
Sources: Adam Hart, Ph.D., professor of science communication at the University of Gloucestershire, in the U.K., and author of The Life of Poo; Bradley Howard Kessler, M.D., director of pediatric gastroenterology at Good Samaritan Hospital Medical Center, in West Islip, New York; Linda Palmer, author of Baby Poop: What Your Pediatrician May Not Tell You.