Oh don't you just Hate that!!

If you have the option (I'm not sure if you have a private OB or not), as the Ob to write a referral for a growth scan.
You should be able to get one done, and then get them to check again.

I had one done at 30 weeks cause I was getting all stressed out about being given 2 different answers, and my Ob decided that it was best to check again for fear I'd get high blood pressure from the stress of not knowing LOL

He said to tell the people it was a dating/growth scan for medicare purposes.