At one week old, if your baby's having plenty of wet diapers that's a good sign he's getting enough milk, but it's worth checking with your PD or the hospital ward you delivered at just to be sure everything's on track. Sometimes newborns can have a slower start with pooping, especially if breastfeeding is still getting established, but your doctor will want to rule out anything else and can give you proper peace of mind. How's the feeding going otherwise - is baby latching well and seeming content?