If your baby is at least 4 weeks old or older you can help a baby relieve there constipation by giving them some prune juice in the babies bottle of milk or breast milk.
If your baby refuses to drink the prune juice in the milk then you can try giving them some apple juice or even some pear juice in the bottle with or without the formula.
Your baby should start pooping eventually if you keep giving them enough prune juice or other juice in their bottle.
If not and your baby seems to be in pain or hasn't pooped in 3 to 4 weeks then you should see your babies doctor.