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Help determining when I conceived/who the father is?

trying to figure out when I conceived and who the father is. My Ex boyfriend and I Had sex March 4th-6th, 11th-13th, 18th-20th, 24th-28th. We pretty much broke up the next day. Then April 1st-3rd I had sex with another guy. Took a pregnancy test April 6th with first response and it was faint positive..also took a digital clear blue and it read "Pregnant". Went to clinic to take their urine test on the 8th of april two days later...it was positive. Then went back for ultrasound May 3rd and they said 7wks 2days. I went to a dating scan the following month on June 8th that put me at 13wks1day, which is 5 days earlier than the US on May 3rd, putting my due date at December 13th. I calculated my conception date from my due date which said I conceived around March 22nd. However, I don't know when I ovulated due to irregular periods. Also, not sure which scan is more accurate..so its just a mess. I don't know if I got pregnant with the other guy on April 1st-3rd or if it was from the sex in march with my ex.
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134578 tn?1614729226
Not the second guy.   Conception would not be as late as April 1, if you got a 7 week 2 day count on May 3 from an ultrasound.  I have limited time and no access to resources at the moment but will write back tomorrow night and go over it with you.
All medical counts of pregnancy (the number like "7 weeks 2 days" or "11 weeks 4 days") begin on the first day of the last period you had before getting pregnant, or a calculated first day of last period (computed from averages of lots of women), which is calculated at 2 weeks before conception.  If a doctor says to you, "Congratulations, you are 7 weeks pregnant!" he means 7 weeks since your last period began, or about 5 weeks since conception.  

The count has been done this way since before the advent of ultrasounds.  The period used to be the only clue the doctors had to try to get an idea of when the woman's pregnancy began.  They would start the count at day 1 of the last period, not because the doctor thinks you are pregnant when you are on day 1 of your cycle (they know you are not, you are having a period!).  But it was the only signal they had.  This count (the GA count) has continued to be used through nowadays.  That's why pregnancy, which takes 38 weeks, is counted as being 40 weeks long.  They add two weeks at the front end, to calibrate it to when they assume the first day of the last period was, and began the count on that day.

A lot of women don't ovulate neatly two weeks after the first day of their last period.  But the ultrasound sees and measures the actual embryo, so for a woman who gets a GA count that doesn't line up with her real period, she should believe the GA count, less two weeks, for dating conception.  When the doc told you that you were 7 weeks 2 days 'pregnant,' he was saying your embryo measured about what an embryo would measure that had been conceived 5 weeks 2 days ago, or about March 27.

The only other thing to remember is that ultrasounds can become less accurate for determining a conception date as the pregnancy progresses, since although all embryos begin life as one cell, some grow more quickly or slowly than others.  An ultrasound at 6 weeks 1 day can be spot-on for dating conception, and then in the same pregnancy an ultrasound at 12 weeks can have a margin for error of +/- about 7 days.  So trust your earliest scan when using scans for dating purposes.
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