You kinda figured you had a good 12 years or so before having to deal with your new daughter’s mood swings, fluctuating hormone levels and the constant stealing of your tampons. But low and behold, lit-er-all-y the day you gave birth and went to change your first diaper, you discovered blood! WTF.

See, during pregnancy, a surge in maternal estrogen levels can stimulate a female fetus’s uterus. Within the first week of life, it’s not uncommon for baby girls to have a mini period in which the uterus sheds a little blood. This baby “period” won’t last long and should only be a small amount of blood in the diaper. “A baby period is due to withdrawing from the mother’s hormones that the baby was exposed to in utero,” says Dr. Meredith Grossman, a pediatrician at the Mount Sinai Doctors Faculty Practice.”It will only last for a day or two. After this, her next period will be when she goes through puberty.”

“A baby period is due to withdrawing from the mother’s hormones that the baby was exposed to in utero.”

Check with your doctor if the bleeding lasts longer than a few days or is accompanied by a foul odor. If the blood in your baby’s diaper seems to be mixed in with their poop, it could be a sign of a food allergy or intolerance, especially of certain proteins in milk. This condition, aka allergic colitis, usually happens during the first few months of life and is accompanied by fussiness and even vomiting. If you experience any of these symptoms, chat with your medical provider on treatment options that may include switching formulas or going dairy-free if you are breastfeeding.