You have not mentioned about type of milk. Breastfed babies are less prone to constipation.
If you are giving formula, then check if it is properly prepared.
If there is no pain abdomen, or distended, or bloody stools, or vomiting this constipation is probably safe.
You may give fruit juice 30 to 60 ml daily to treatment
No laxative
Next Steps
consult a doctor if symptoms described above or constipation doesn't improve.
Health Tips
breastfeeding is best for babies.