measurements?!

I went to the hospital last night due to a UTI and they checked baby did all the measurements to make sure he's ok and I'm suppose to be 17 weeks today but my I was watching him and my baby's arms and legs were measuring 15 weeks is that normal? He's not suppose to be measuring 2 weeks behind the doc said he was only measuring 1 weeks behind at my 9 weeks u.s and he said that was normal for baby to not quite catch up to his weight yet