I went through this as well and called my antental nurse in a panic after seeing the midwife at the hospital (doing shared care). Every doctor, midwife, nurse will come up with different measurements, depending on how they hold the measuring tape and where on your pelvis/belly they measure to/from. On the weeks that I was measured by my antenatal nurse at my GP clinic I always measured exact to dates, but when being measured at my hospital visits by midwives I always measured smaller. Also as baby moves head down and starts to drop down into your pelvis a bit more your measurements tend to get smaller or stay the same as the weeks pass.
At 39 weeks I was measuring 38, and yesterday at 40 weeks (with a different midwife) I was measuring 36. They weren't concerned at all.

Of course, if you are still worried ask about it