My ultrasound said my due date is august 29 is it 100% I got pregnant in december I need to know its guaranteed! Can you help me here thanks?

Buy a car. If you want a guarantee, buy a car! since you were given a due date of aug. 29, you had to have had a very early fetal study, those dates would make you 6wks 4dys as of today (1/7). Estimate should be accurate to less than a week. Estimate of conception would be around dec. 6. Now, if you actually conceived 4 days earlier and had sex with undesired father nov. 30, it is barely possible he's it!
Not enough info. Ultrasounds are more accurate for establishing the due date when performed in the first trimester. With a margin of error of 1 week. If done in the third trimester the margin of error could be 2 to 3 weeks.