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Having irregular period. Got pregnant. How to calculate conception date?

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Practicing since : 1999
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trying to work out conception date and unsure if i had a period or an implantation bleed in December or not?
I just need clarification really, I am fairly aware that it is near enough impossible for this baby to be the man from November, but i just cannot have this child, if there is even a little bit of doubt.
Also, right now I am around 13 weeks 1 day pregnant (I think, I have not had a dating ultrasound yet)
My cycles are usually about 29 days long and GENERALLY quite regular, lasting about 4- 5 days of a quite medium to light flow
In November 2012, I had a late period, it was about 4 days later than expected (i believe), so I had a period on 13th November 2012. I had unprotected sex on 23rd November and quickly got a new boyfriend around the 27th December 2012, we had alot of unprotected sex from this time until my next 'period'.
Ok, so I was expecting my next period to be due on 11th-12th December, instead it came on the 8th December, and it was really strange, it was not like my normal period, it started off like spotting (it varied from dark brown to pink) and this lasted about 2 days on and off, from what i remember, there was one day, when i did actually bleed like a light period for a few hours, and this was immediately after sex, and it was a fair ammount and quite watery (possibly from the intercourse?). However, overall this 'period' was enough to concern me and so i took a pregnancy test (i really can't remember when) but i think it was around the 12th December? possibly earlier, and the test was negative.
Bear with me, I know this is quite long.
Ok, so, we continued to have lots of unprotected sex into December and my next period was expected around about the 5th January 2013, it didn't come and then on the 7th December i took a test and it was a quite strong positive.
I considered abortion, because I am young and my job does not really pay enough etc, and I had an ultrasound on January 21st 2013, the doctor said I was 5 weeks and 5 days and we somehow calculated a conception date of 12th December?, which would just be impossible. I wasn't sure if the doctor had already added 2 weeks onto the actual age to make it match up with my last missed period or not though?
I then had annother ultrasound on 13th February 2013, and was told I was 9 weeks and 3 days, which seemed along track, again though, did the lady add on the 2 weeks to make it match up? meaning i was technically 7 weeks and 3 days?
I am really confusing myself and hopefully have not confused you. All dates are correct, because i recorded them, unless stated otherwise.
Does this information point towards a pregnancy concieved around the 23rd November? ( I should have ovulated not long after i had sex, I think?)
Or does it point to one in the middle of December? And could you possibly help me work out the date please?
Thankyou so much in advance.
Posted Tue, 2 Apr 2013 in Pregnancy
Answered by Dr. Timothy Raichle 45 minutes later
Hello, I would be happy to help you with your question.

I am going to pretty much ignore your cycles, because they do nothing to confuse the situation. Ultrasound is remarkably more accurate in terms of establishing when conception and growth began. Here are the facts (EDD = due date):

1. Ultrasound 1/21 placed you at 5 5/7 weeks
EDD = 9/18, conception around 12/26 (+/-3 days)

2. Ultrasound 2/13 placed you at 9 3/7 weeks
EDD = 9/15, conception around 12/23 (+/- 3 days)

So taking into consideration the error in ultrasound AND the fact that the ultrasound #2 is probably a little more accurate (because it is easier to measure a fetus that does not look like a ball of fuzz), I believe that you conceived around 12/20-12/26. Somewhere squarely in that window. Once you have the ultrasound, you can forget the confusion surrounding your cycles (though the LMP of 12/8 would fit with conception right around 12/22). So, I would place your due date as 9/15 as well.

I hope that this helps. Please ask if you have any followup questions.
Above answer was peer-reviewed by
Follow-up: Having irregular period. Got pregnant. How to calculate conception date? 17 hours later
Thank you, that does help
So this is a pregnancy from the middle of December, and not the end of November/beginning of December then?
Also, the weird bleed I had on the 8th December, do you think that was my period then?, what are some reasons, other than implantation bleed, which would cause this type of 'period'?
Answered by Dr. Timothy Raichle 2 hours later
In answer to your first question, and based on the ultrasounds performed, the conception of this pregnancy was right around 12/22-12/23 - there is little doubt about this.

I am not sure what to make of that bleed. At this point it is somewhat irrelevant because, based on how far along you are, it was not related to being pregnant!!

If you have a followup scan, please feel free to run it by me here by requesting me and I will review all of these dates with you again.

I hope that this helps!!
Above answer was peer-reviewed by
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