Hello,
I need some help estabilishing a more accurate conception date.
I had sex with a man around the end of march, like march 25 or 26 , he did not come inside of me, we ended up finishing with oral sex, that's how he came.
Then my period came a little late, like 5 days late, and also found out through a ultrasound I had an ovarian cyst. (that went away with my period though.)
Just to be sure, I took pregnancy tests and they were all negative then.
So my period came around April 3rd, and lasted like 4 days, like it has always been though.
Then around April 9 I had oral sex, no penetration at all.
From April 13th on I started having unprotected sex with my boyfriend .
On may 17th I found out I was pregnant, and the same day I had an ultrasound that was stating I was 6 weeks and 3 days pregnant and the due date was January 7th ,2013.
The other ultrasounds I had were stating that my due date was on January 10th, ( morphologic ultrasound I had at 20 weeks, and a 3d one that I had at 25 weeks)
I need to know when I have conceived. My doctor says sometimes around April 16th.
I need to know if there is a chance that I got pregnant , maybe with pre ejaculation sperm, at the end of march,and maybe what I thought it was my period, was only implantation blood?
If so, though, all the ultrasounds I had should have stated that I was 2 weeks ahead, right? Please answer me, I'm very anxious!

Ultrasounds are a best guess, they can be off by as much as 2 weeks either way, so a month.

There's no way for anyone, even a doctor, to tell you the exact date you conceived. If there's a 2 week discrepancy in partners, it could be either one.

The only way to accurately determine paternity is with a DNA test after the baby is born.

I thought the early ultrasound could be very accurate.. 2 weeks off it's a lot of time.. shouldnt they have told me that I was two weeks more?

No, they according to normal size of fetus and normal development on the average person, they give you a specific date. It is seldom correct exactly.

And they CAN be off up to two weeks, but often are within one week,

And no they don't tell you they are not exact.

So there is still a chance that : instead of being 34 weeks now, I'm 36 weeks, and also the menstrual period wasn't a real one, and so I conceived at the end of march instead of middle of April?

So there is still a chance that : instead of being 34 weeks now, I'm 36 weeks, and also the menstrual period wasn't a real one, and so I conceived at the end of march instead of middle of April?

Yes.