Green poop in a 20-day-old baby may be caused by improper diet, or it may be related to indigestion and other factors. 1. Improper diet, if usually breastfeeding, the mother often eat food containing more pigment, may lead to pigment into the baby’s body, so there will be 20 days of the baby’s green poop symptoms. 2. Indigestion, if you feed your baby too much at once, your baby’s bowel movements will increase after indigestion, and you may experience green poop symptoms. There may be other reasons why a 20-day-old baby has green poop, so it is recommended that you go to the hospital in time for a complete examination to determine the cause of the problem, and then give targeted treatment under the guidance of the doctor.