So I'm officially an August '15 Bump??

Ok ladies, my original EDD was Sept 8 2015. Going by my lmp which was Dec 2 2014.
 I just went to my follow up scan because at my 20 week scan (that was taken when I was 22 weeks) there was a head to abdomen ratio of 1.03 that didn't sit well with my midwife. She just wanted to make sure his head was significantly bigger than belly. 
So today's scan (I am 28weeks 5 days) showed a greater ratio and my u/s tech said that had to be a good thing. But my son is measuring at 3.8 lbs and that puts me at a new due date of August 24 !!! That's 15 days ahead! 
Will my midwife officially change my due date ? How reliable is his info? And has anyone had a weird head to abdomen ratio and everything turn out ok?