Pregnancy Timeline: Estimating Conception Date & Gestation Age

Question
I am 22 years old and this is my first pregnancy. I think the first date of my lmp was June 12,2004 but i'm not completely positive. it was a short period. according to my lmp i am now about 19weeks 3 days pregnant but my ultrasound says i'm 20 weeks 3 days pregnant. I had intercourse with my ex-boyfriend on June 4 and had intercourse with my now boyfriend on June 19-27. Is it at all possible that i could have conceived on June 4? Me and the ed boyfriend used condoms but me and the new boyfriend used none because he had been told that he was sterile. please help me figure out when i could have conceived.
L.S.

Answer
If the sonogram measured you at 20 weeks 3 days, then your last period started June 8 and you could only have conceived if you had unprotected sex on June 20,21,22, or 23. You could not have conceived on June 4 if you got your period after that (June 8). If your last period started on June 8, you would now be 20 weeks 3 days (from the first day of your last period). If your last period started on June 12, you would now be 19 weeks 2 days. The sonogram has an error of +or- 2 weeks at this time, so I would agree with your dates and say that you conceived on one of the days I mentioned above, and you are due on  March 14, 2005. Your new boyfriend is the father of the baby because you did not become pregnant on June 4.