Measuring 4 weeks ahead

Cherise
Im currently 33 weeks, but measuring 37. Had an ultrasound today, and the baby is within wt. range, amniotic fluid is normal. Only one baby still seen. I dont go back to my OB for two weeks, any ideas what it could be. Baby appears healthy in all ways that can be seen. I do not have diabetes., high blood pressure, or any of those medical complications.