If baby is alert and happy, I wouldn't worry about it. However if baby is lethargic or seems to be in pain (curling up into a ball, hard stomach, etc.) then you should contact your pediatrician right away.
My baby went 10 days without pooping at some point. Her pediatrician prescribed some suppositories (OTC). The suppository worked every time, but I have to warn you it really hurt their tummy.