A 40-day-old baby with green poop may be in a hungry state, or it may be due to bile secretion or iron supplementation. 1. Hungry state: If the baby is not full and is more hungry, the peristalsis will increase and the mixture of undigested bile may be manifested as the baby’s green poop. 2. Bile secretion: fat decomposition, absorption process, bile will play a role, excess bile will be discharged with the stool. It may be manifested as the baby’s poop showing green color. 3. Iron supplementation: milk powder may contain more iron, some babies may have anemia due to their own factors, etc., and need additional iron supplementation. If the iron cannot be fully absorbed, the excess iron will be excreted with the stool, which may be manifested in the baby’s poop showing green color. If your 40-day-old baby has green poop, it is recommended to observe closely and seek prompt medical attention to confirm the causative factors and follow the doctor’s instructions for symptomatic intervention if any abnormality occurs.