How accurate is a ultrasound for the conception date if they tell me that I'm at least 32 weeks pregnant. When I count back the weeks something doesn't add up because I wasn't with anyone during that time. Just 3 weeks before and a week and a half after.

Short answer is not that accurate. They tend to go on the size of the baby, but babies do vary, and that can affect the dates. Go by your last period and/or when you know you had sex.

@ Eileen G - I've always had irregular periods and not sure when my last one was I think it might have been the month before. But I'm concered to who the father is and which conception date to go by? I am a heavy set girl who has always been irregular.

Also I had just found out that I was pregnant in my 3rd trimester and I've read online that an ultrasound done in the 3rd timester can be off by 3 weeks either way. So does that mean it could be the guy from the 3 weeks before or the guy from the 1 week after? I'm so confused?

In that case, I wouldn't try to guess who it was. Are you sure about the dates you had sex with the two different men? If they were a full month apart, then you can probably take an educated guess when the baby is born, depending on whether it looks a bit early or late. But trying to guess the father from the ultrasound is as reliable as tossing a coin.

If this is a question of paternity there's only one way to know for sure, and that's a DNA test after the baby is born.