Here's my theory that I've never bothered to ask about but I bet it's true.

You know how babies out of the womb have growth spurts? Where in one week they can be 300g heavier even? I reckon babies in utero do the same thing, because in my pregnancies I have measured behind, ahead, behind, ahead...

Or it's possible that bub is just a titchy one. A midwife noted that I was pretty small for dates this time and asked how big I thought bub would be. And he was 7lb9oz (compared to his 9lb4oz older bro) which isn't too small, but at 1 year old he's pretty small now. Perfectly healthy. Just...compact!

If your ob says not to worry about bub's size, then don't worry.