What can I give to my 4 mth old baby for constipation?

My baby girl hasn't done a poo in about 6 days and she seems to wants to do one. But it looks like she is in pain trying. She is fed. I was told that a baby that is fed can go upto 10 days without doing a poo. But I would like to know what I could give her to help her do a poo?