I know when my lmp was but when they didn an ultrasound they gave me an edd of january 13 when it should be around december 31st?

Confirm due date: Based on your information, the edd is 1/13/2018 based on ultrasound. Gestational age today: 18wks5days Most likely date of conception : 4/22/2017 If your initial ultrasound was performed in the first trimester, it probably will give you the best date with +/- 3 to 5 days and I will choose this over the edd based on lmp. You doctor need to make a conservative decision based on second trimester.