Is there a huge difference in a 10w and a 14w u/s? i had a u/s that dated me 10w2d, is there any way it could be off by 4 weeks? when did i conceive?

Huge Difference: There's no mistaking a 10 vs a 14 wk fetus on ultrasound. Even a rookie sonographer wouldn't make a mistake with this. If you were 10w2d today (12/23), you conceived around 10/26. A 10 wk fetus is approximately 1.2 inches in length vs a 14 wk fetus which is approximately 3.4 inches in length... Almost 3x the size and you can see a lot more detail. I hope that helps, you're off by a month.