Understanding Potential Pregnancy After Unprotected Sex & Cycle Disruption

Question
Hi Brenda. I hope you can help me, I will give you all details and facts. On the 19th of June approx 5am I allegedly had unprotected sex with some1,I know this sounds bad but I cannot remember as I was VERY intoxicated and only remember falling asleep.Anyway on the 20th I had my period as normal. on the 26th my ex partner of 6 years got back together and i didnt restart taking my pill. during the w/c 13th(i think it was the 16th) July I had a negative pregnancy test and on the 21st July had a positive pregnancy digital test saying 2-3weeks. at 6weeks and 3 days from lmp i had bleeding/pains and was sent for an early scan which I was given 2 pictures of and they measured 1=2.7mm and the other 3.9mm. my midwife says this would be correct to have concieved after the alleged activity on the 19th June. Yesturday I had my 12 week dating scan and I was told i am 12+5weeks(2days extra) but measured 61 which is the size of a 10 weeks fetus. what im confused about is that it would be 12+4weeks since the 19th June. so does that mean the conception is from the 19th or after.i also used one of those conception calenders and it says for my due date being 25th march i would have concieved around the 29th-4th July. how accurate is this. im so confused about this and really thinking about having an abortion as this wis something that could ruin alot of lifes if it is from the 19th June. I really dont want to but I cant take the risk. sorry if any of this sounds confusing for you and thank-you

Answer
Dear Claire,

OK - unfortunately, the information is rather confusing and I've read through it twice. :-) So, here's the thing... It would have been very unlikely for you to have conceived on the day before a regular period started if it was a normal period and you have a history of regular menstrual cycles. Not impossible - but unlikely. IF, repeat, IF you had conceived on that day before your normal period on 6/19, an ultrasound this week would have dated you between 14-15 weeks. If the scan this past week is dating you at/about 12 weeks, that corresponds with a conception date on/about 7/4/09 and a LMP date of 6/20/09.

Here may be the source of your confusion...
The calculation of gestational age, or pregnancy dating, is not actually based on the conception date. It is based on the LMP date. Always. Even if the LMP date is not known and even if the dating is determined via ultrasound. The ultrasound will automatically calculate the gestational age determined by the measurements of the fetus or gestational sac, etc,..., but that is all based on norms for gestational age calculated from LMP. The actual "full-term" confinement of pregnancy is 40 weeks from LMP to delivery. It is assumed that conception takes place two weeks after the LMP date because most menstrual cycles last 28 days, with ovulation occurring on/about CD 14-15. I know... this is very confusing for women, but that is just how it is.

I hope this has helped you and answered your question. I wish you well.

Brenda