Progesterone in 1st tri?

BananaPancake

Hi Mamas - thought I'd start here. My OB called today and my progesterone was at an 8 at 6w. I'm pg with twins and they are starting me on 200 mg prometrium 1x a day. Can anybody weigh in on this dosage? We've seen the heartbeats and they weren't worried about HCG at all, of course, just wanted to get me started on progesterone as a "preventive measure." 

I didn't have bloodwork until about 9w with the rest of them, so they think I might just always have low prog and carry babies fine, but I'm still freaking out.  

 Can anyone give me some encouragement, here? Thank you!