Firstly as it implies that only your first baby is precious and all subsequent ones are dragged up... without the book
Secondly, that is used so negatively far too often and used against people who are actually, just concerned about their child, or about their parenting decisions- regardless of their place in the sibling queue. It simply is not relevant.
Lastly, the way that it is used so patronizingly. With the implication that just because you have only one child ( so far...) that you have no common sense or experience with regard to bringing up children.
OK, I know it is also used in an endearing way too. I just desperately dislike it. It really annoys me.
erm... that was all really!