Breastfed babies sometimes go for ages without doing a poo. My daughter went for more than two weeks without once, when she was around six weeks old. Took her to the doctor and he said as long as she didn't seem uncomfortable not to worry too much.
I think it's partly because breastmilk is a really efficient food, and there's actually not that much waste.
My DS did this and the periods between poos got longer and longer. I too was worried because it's not what you think a baby should be doing,(I thought he'd be pooing every 5 minutes)! Good luck with the nappy when it does come, it can be like nothing on earth!
Another wee breastfed baby who causes poo trouble.
My nearly 6th month old only goes once a week at most, and sometimes 10 days. He does not have a hard tummy and is not in pain, in the day just before his about to go, lots of stinky trumps and a bigger belly than normal, then tonnes of poops.
Ive worried and searched the internet and loads of breastfed babys do it.
As long as tummy is not hard and not in pain, not a problem.