23 or 26 weeks???

Kelly
The doctor gave me a due date of January 13th I am currently 23 weeks. I went in for another ultrasound today and the baby is measuring 26 weeks. In three weeks I go in for my glucose screening then in three more weeks I go in for another ultrasound. The doctor said at that point he will reevaluate my due date. Is this normal? Has this happened to anyone?