Up to a month? Really?? I don't know how old your child is, but my daughter went 6 days without pooping when she was about 3 months old. My ped said that they don't want them going longer than a week. After that, there is concern there could be a blockage or something more severe going on.
We gave her prune juice mixed with water per my ped's direction and it did the trick. The next day she was pooping like crazy. Try 1 oz of water mixed with 1 oz of prune juice. Again, not knowing how old your baby is, I'll tell you that at that age (3 mos), they can handle up to 4 oz a day. Obviously, the older the child, the more they can be allowed to drink.
I'd get a second opinion if I were you. A month just sounds weird. Would you go a month without pooping and think it's normal???
Good luck!