belly measures big? & I lost my mucus plug!

My due date is 12/12 based on our first ultrasound. My LMP would place the EDD on 12/4- not a huge difference. 
Well my entire pregnancy the baby has measured two weeks bigger... I have type one diabetes so it was to be expected. We never changed my due date though.
Well in my last ultrasound and doctors appointment my belly measures 39 weeks and I was only 31. My amniotic fluid dropped by half in about 4 weeks and she weighs about 4 pounds. We're going back now once a week for an ultrasound to check growth, but I'm just worried that my belly will stop growing and think it's time to deliver! Anyone have experience with this?
Also, I lost my mucus plug (31 W 3 d).