I just wanted to add, both my boys had big ones at 4mo (ds#2 lasting nearly 3 weeks... but he was quite ill just before too/ ds#1 was at least 10 days) But to the best of my knowledge neither had one at 6mo... I think the thing to remember is that our babies don't read the books & also that to have an "average" there has to be a 'range' that is normal.