Earlier ultrasounds are the most accurate. Conception is an estimate as it all depends on exactly when you ovulated, (could have been late) and length of your cycle. If you had an exact 28 day cycle and ovulated exactly on day 14, you would have conceived on or around the 25th of August, however, not many women have an perfect dates, that's why they give you a window of conception.
Unless you were tracking ovulation and know exactly when, there really is no way to know the exact date.
roughly you conceived within 2 weeks before your period to 2 weeks after your period, and yes the first ultrasound is usually the most accurate nunless you are too early then the docs take a second one around 8-12 weeks gestation to get a better idea of baby and edd.
ah this is all so confusing haha. i dont think i ovulated on time because if i did then my due date wouldn't be may 27th it would be a week before...so i guess theres really no way to tell. my first ultrasound though only showed a gestational sac and was done at savalife where they have low quality machines, so maybe that had a lot to do with it.