Anatomy scan scare... Help

Jazmin • Mother to an angel... I love you Adelyn Rae Bower 5.8.15
I posted about this a few days ago but didn't have any responses... But there has to be someone who's either going through or has gone through what I've been going through. 
At my anatomy scan at 20 weeks they told me that she was only measuring at 17 weeks and 6 days.
Can that just be that she's going to be small? Or a premie? 
The thing that scares me the most is that when they looked at the brain they thought that the left and right hemisphere may be "fused together" but I feel as though they were just having trouble finding a good picture because she was just so bunched up and they couldn't seem to get any measurements that they needed.