Expert: Dorothy H, LCCE - 6/28/2010

Question
"Hi. I am going crazy trying to figure out how pregnancy, conception, intercourse dates, and due dates are calculated.  My LMP was 12/19/2008, which lasted 3-5 days, that's the average length, 3-5 days. I had sex on 11/28/08, 12/7/08, 12/25/08, 12/29/08, and 1/2/09. My son was born on September 25, 2009, his expected due date. Given the above information when is it mostly likely I conceived?  Is it at all possible that I conceived on 11/28/08 or 12/7/08? I had an early transvaginal ultrasound on 2/16/09, that dated my pregnancy at 6weeks and 4days. Which made my expected delivery date 9/25/09. I look at some sites that say you should deduct 2 weeks from the amount of weeks your pregnant to figure out the conception date? Is that true? Because they gave me a conception date of January 2, 2009, should I deduct 2 weeks from that date as the "actual" conception date? I thought conception date was the same as intercourse date but now I see sperm can live 5-7 days so that's not the case. I'm just confused. Can you break it down without the 2 weeks added so I can know for sure when I conceived based on when I had intercourse? Thanks so much for your time."

Answer
Hi Jay,

Yes it can be confusing!

A transvaginal ultrasound in the first trimester could be off by 2-3 days, but that's it.

It is NOT possible at all that you conceived 11/28 or 12/7.

You conceived around January 1.  Yes, sperm can live 3-5 days (7 is theoretically possible, but 3 days is more realistic) so giving a 3 day window for the ultrasound,  the egg was released Dec 29-Jan 4.  We'll give the sperm 5 days - so count back another 5 days to Dec 24.  (And add on an extra day at the end, because the egg lives for about a day.)

So that would mean that the time period when you could have gotten pregnant is from December 24-January 5, which means you conceived one of these three days:  12/25/08, 12/29/08,or 1/2/09, with the most likely dates being 12/29 or 1/2. I can't pin it down more than that - 12/29 and 1/2 are equally likely to be when you conceived.

I hope that helps!  

Dorothy H, LCCE