My cervix was measured at about that, with funneling, at 20 weeks. I was put on strict bedrest and was in and out of the hospital for PTL as well.
I was told that they don't lengthen significantly...most of what appears to be lengthening/shortening can also be considered general error in measurement (if that wasn't a transvaginal measurement, it will be less accurate and even then different people will measure differently)...all you can do is hope it doesn't get worse. I wasn't a candidate for a cerclage (its debatable for twins, and I had a history of bleeding). My girls did come early, but feel free to PM me if you wish.