Babies can usually go up to 4 to 5 days and in some cases up to a week without having a bowel movement so it's very normal for a baby to not poop for up to a week.
But if your baby hasn't pooped after a week then they're likely constipated. Your baby might have a grunting face when he/she is trying to have a bowel movement and they're constipated.
My nephew had that funny looking face when he was 5 months old and was constipated but couldn't poop.
It was funny when he made that face but I know he was having a hard time of pooping and it wasn't funny for him.