Cass, I was told by the midwife they just use this measurement as a rough guide and its common for it to be out by a couple of cm's, because it can change depending on the position baby is in at the time. They said if they can hear bubs heartbeat and you can feel movement then the fundal height being a couple cm's less than your number of weeks is no issue.

What a cute Belly picture too! I'm sure your bubba is just fine.