Conception with Irregular Periods: Understanding Your Due Date & Timeline

Question
I am 20 years old and this is my first pregnancy. I don't know how to figure my conception date because I have never had a regular period. I have had ovarian cysts and I don't know if that is the only cause but I usually get my period about three times a year. I can't remember when my last period was but I know is was this past summer in June or July. My due date keeps changing with every ultasound I get and the doctors say that is could be a two diffrence. At my first ulta sound on Sep 25 they said I was 7 weeks and 2 days with the due date of May 5 that was at a hospital. My next ulta sound was with my doctor and they gave me a due date of May 3 and then my next ultasound gave me a due date of May 10. I need to know the conception date but it seems almost impossible and the dates they give me make it right between two of my boyfriends and this is killing me. My exboyfriend was unpretected and my current boyfriend was protected but i know that is not always a hundred percent. With this information can you figure anything out for me?

Answer
A sonogram is not accurate at 7 weeks gestation. It is more accurate at 12 weeks and very accurate at 20 weeks gestation. What was the date of the second and third sonogram that you took. If the three sonograms showed that you are due between May 3-May 10, then your last period probably was between July 28-August 4. That would mean that you probably ovulated around August 10-18. You could have conceived if you had sex on August 9-20. Thats the closest I can calculate with the data you gave me. If yuou had a sonogram showing you to be 18-20 weeks, that would be the most accurate and you should go by the due date of that sonogram.