It could also be the baby's position while you were getting measured, the person measuring, and a bevy of other factors. My best friend always measured 5 weeks behind. She is tiny. I always measured 4 weeks ahead. I am not tiny
I am really short waisted and always measured ahead with dd after the midpoint of pg. She had normal fluid levels, was of a good avg size at birth, no genetic anomalies or anything. I think women's bodies are way too different to hold them all to a centimeter mark!
like all the previous posters, i too measured big. 4 cm above by the last 2 months. i was worried of course, even though my midwife wasn't. my baby was born easily at 8lbs. every woman grows a baby in her own way!
I measured 4 weeks ahead with DS1, and 1 week ahead with DS2. DS1 arrived at 38w, 8lbs 4oz and 22" long. DS2 arrived at 39w, 8lbs 6oz and 21" long. As long as you are growing at a nice steady rate, I wouldn't read much more into it!