Very curious..

[sahm_03]

Hi.. Im 9 wks and 3 days pregnant according to a dating scan I had at 8wks 1 day but 10 wks and 2 days according to my LMP which was the 6th of may. So theres a 6 day differance there. I do have a longer cycle than 28 days though - it varies between 30-34 days. I had a one night stand on the 11th of may and sexual intercourse with my ex partner on the 20th and 25th of may. My due date according to my scan is FEB 16 2014. what date do you think I concieved?? II'm confused because when i count back 9 weeks 3 days, I dont get my lmp date of the 6th of may, I get the 12th of may which was 1 day after intercourse of the one night stand. Im thinking I may have ovulated 6 days later on day 20 to account for the 6 day difference of gestation between my scan and LMP... ??

[sahm_03]

Thankyou very much. That has eased my mind immensley.

[AnnieBrooke]

Don't try to account for the difference, when you have a different-than-average cycle length.

Ovulation works like this:  You have a period, some time passes (and for some women, this can be a lot of time), ovulation happens, and 14 days later a period comes.  If someone's cycle is an irregular or unusual length, it is at the front end, it will still be two weeks after ovulation when her period comes.  Therefore, when your actual period began doesn't matter to the ultrasound that dates the pregnancy.  If you were told you were 8 weeks 0 days when you had your ultrasound, you were 6 weeks from ovulation and conception at that time, even if 8 w 0 d does not match up with the first day of your last period.  

Don't waste time puzzling over discrepancies between first day of actual last period and the one projected by the computer in the ultrasound and trying to extrapolate forward or strike some kind of balance between the two.  The ultrasound saw and measured the actual embryo that day, and calculated forward to your due date because of the size and various development milestones of the embryo on that day.  It wouldn't matter if your first day of your last period was one, seven or seventy weeks prior, it knows how big and fully developed the embryo was on that day and projected forward to an estimated due date based on that information.  From that projected due date, you can reliably count back 266 days to get to the date of conception, whether your period began 280 days earlier or not.

For what it's worth, I get around May 26 for ovulation and conception.  The May 11 event seems quite a bit too early.