baby measuring 4 weeks ahead

At my 27 week apt I was exactly 27-2 and the doc measured my girl and told me she's measuring at 31-4 and give or take a few weeks on my due date now. My original due date is march 27. I'm now 28 weeks pregnant. Has anyone else encountered this and did their babies come early? I'm going to be prepared for her arrivival anytime between the end of feb and end of march I just want to know how common it is!!