due date with twins?

So my original due date was on Dec 15th then based off the ultrasounds the doctor changed it to the 22nd and upon another ultrasound changed it to the 21st. Reading most Twins only go to about 35 weeks that would put me on baby watch around November 16th. I have PCOS so I figure maybe I just ovulated late in my cycle causing my due date to not be accurate? But then how to figure when your due date is when Twins typically come early? I also have history of Preemies (no micro 34 and 35 week gestation on my singletons) I am just worried they will come too soon... is the general goal with Twins to keep them cooking as long as possible? I haven't seen the specialist yet but have the OB working on the referral. I am just a worry wart I guess.