head measuring big, abdominal small

Kim
Hey, I am 30 weeks 3 days pregnant ... 
So I was in hospital being checked over for reduced fetal movements and the doctor decided to do a growth scan. The midwife let me go home afterwards said we need you to come back in 2/3 weeks for another.
When I've checked the measurements from my file it says that the babies head is measuring 4 weeks ahead but abdominal is 2 weeks under.
The midwife never told me the reason that I need to go back so now I'm worried that the babies head is too big, or the abdominal is too small? 
Has anybody had anything similar? Also I wasn't tested for diabetes as my bmi was slightly under, is this something I should push for testing? As I have read this can cause big babies/measurements 
Thanks in advance for any help X