Are the first ultrasounds pretty accurate in determining conception date?

[loridarlin17]

I have a serious question that has not been bothering me until my boyfriend was bothered.

On July 12-14 I had my last period
Then on July 24th I had sex with my (now) boyfriend
But then... on august 18 I had a one night stand with another guy.
I found out through 3 home pregnancy tests that I was pregnant almost 2 weeks later.
Note* I did not have my august period.
Got an ultrasound 2 weeks after and it said I was 7 weeks 5 days pregnant, and the doctor said he thought my conception was July 26th, which is only two days off from when my (now) boyfriend had sex.

My now boyfriend is concerned because he thinks it could be the one night stands baby. Everything points to my (now) boyfriend being the father but it has been bugging me and I'm hoping for some advice on the ultrasound being accurate and if it could be almost 4 weeks off?
We got one of those Prenatal tests done by DDC, just waiting on the results. my heart and my mind says it is my (now) boyfriends.

[AnnieBrooke]

Can you be more specific about what 2 weeks later followed by 2 weeks after means?  In other words, you had a home pregnancy test two weeks after you had sex on August 18.  Does that mean, September 1?  And then "2 weeks after," you got an ultrasound that said 7 weeks 5 days GA.  Was it two weeks after the September 1 test or two weeks after the sex on August 18?

You're right that things point to your boyfriend, and the simplest thing to do is wait for results on your DDC test.  I can't check the logic of the story, not knowing on what calendar date you got 7 weeks 5 days.  

[loridarlin17]

I apologize it wasn't clear, I meant I got a home pregnancy test two weeks after the one night stand, then two weeks after the home pregnancy test I had an ultrasound. I cant remember those exact dates.

[loridarlin17]

I am pretty sure my ultrasound was sept 1st if I remember correctly.

[loridarlin17]

im sorry, I meant sept 12th

[AnnieBrooke]

If you have the ultrasound, you should be able to look on the face of it and see a date, along with the 7 weeks 5 days GA figure.  Is that where you got September 12?  I hate to give advice on these things and then have the person come back and say, "But that can't be right, because of the ultrasound I had on August 22" or whatever.

Anyway, the ultrasound from September  (whether September 1 or 12) seems to indicate that you got pregnant from your boyfriend, if the only time you had sex with anyone else was August 18.  If you got pregnant that late, you'd only be at about 12 weeks 5 days pregnant now.