At 20 days old, it is normal for babies to still be learning how to digest their food, whether it's breast milk or formula. It is recommended to prioritize non-pharmacological measures for constipation in infants, such as offering small amounts of water and doing leg exercises like bending their hips or simulating a bicycle motion. You can also try safe products specifically designed for newborns. If you have any further questions or concerns, don't hesitate to reach out for more information and guidance.