After about six to eight weeks of age, lots of babies start pooping very infrequently. It's because breastmilk is digested so efficiently. It's entirely normal and as long as the poop is soft when it comes, and there are no other red flags like poor weight gain, there's no problem. It doesn't happen to all kids-- my DS, for example, was an every-fourth-day guy, but his twin sister continued to "go" four or five times a day all the way until she was weaned. Both have their advantages and disadvantages.
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This doesn't concern the OP, obviously, but it's worth putting it out there that infrequent pooping in a newborn is not a typical or expected pattern-- if a baby under about six weeks isn't pooping several times a day, that can sometimes be a red flag that there's something going on-- that for whatever reason baby isn't getting enough milk. I wouldn't dismiss infrequent pooping in a young baby-- I'd be watching weight gain carefully, and looking for ways to increase baby's input, with the help of a good supportive and qualified healthcare provider.
