fundal height with a tape measurer is never accurate. If it was through ultrasound then your doctor should have talked to you about it. Im 30 weeks and at my last ultrasound last week I was measuring exaxtly a week behind. It put my rate at 8% and apparently anything below 10% is cause for a little concern. Mine could be because I have extra fluid in which case im going back so theg can remeasure in 2 weeks to make sure there is progess. But if via ultrasound you are measuring a month behind that doesnt sound good to me. Id call your doctor and ask whats up.
Exactly what I was going to ask. Are they basing the statement that you are "measuring small" on putting a measuring tape on your belly? I always wondered how they affirm each time they measure you whether they are putting the tape on exactly the same spot each time. (It's not like they mark you with a Sharpie at the top of the pubic bone and the navel and always use the same marks each time or anything.)
Are you measuring 27 weeks based on fundus length or have they done an ultrasound to confirm that. I had the same problem. My fundus showed me at 27 weeks and I was 32 weeks along. Ultrasound showed she was perfectly fine.