To help relieve constipation in a 5-month-old baby, you can try increasing their fluid intake, especially if they are breastfeeding. Offer water in small amounts during meals and include foods high in fiber in their diet, such as prunes, oat and barley cereals, and sweet potatoes. You can also gently massage the baby's belly and encourage them to "bicycle" their legs while lying on their back. If these methods do not work after a few days, you can consider using a gentle laxative as a temporary solution before consulting a doctor. Remember to always follow medical advice when using laxatives with infants.