Constipation

Talk to your baby pediatrician first … in my case Glycerin suppositories is what my baby pediatrician suggested for her and it helped her so much

I have the same issue. My doctor told me If you only Breast feed try drinking and eating things that make you use the bathroom. If you drink prune juice it will transfer to her in your breast milk and eventually make her go.

Are you breastfeeding? It's not necessarily constipation unless the poop is hard when the baby goes.
My daughter took a month to start going more consistently. She was going every 6 days for the first month of life.
Even after, it's sometimes normal for breastfed babies to only have a bowel movement once or twice a week. I confirmed this with my pediatrician.
If they are truly constipated - hard stool and struggling - I was told by my pediatrician that a half ounce to an ounce of prune juice in their milk would be ok. I probably wouldn't do this unless the baby was truly struggling with constipation badly. Otherwise, they make glycerin suppositories for infants. Speak with your pediatrician first, though.