Hi, me28715, I assume you know that over time in a pregnancy, ultrasounds get less accurate for purposes of dating the pregnancy to the point where by the final weeks they can be off by two weeks or even more. (Though babies all start out the same size, they grow at different rates.) For that reason, if the ultrasound at 25w3d was your first one, it is no more than broadly useful as a dating tool. (I.e., it can narrow down the month but certainly not the day. The 8-day discrepancy you've named could just be the standard margin for error in action.) Did you happen to have any other ultrasounds, perhaps earlier in your pregnancy?
That said, you are worrying about an event that was more than a month before you would have gotten pregnant. It's not like it's even close. Along with the wide gap in time, you also had a period more than two weeks after having had sex with Mr. Wrong. Women who are pregnant don't commonly have periods, no matter how often someone posts on the Internet a story that someone they know once told them that they had an aunt who had periods all through her pregnancy.
Here is what the conception calculator gives based on the due date of October 16:
First day of last menstrual period: January 9
Probable date of ovulation: January 23
Possible dates of conception: January 19 to 27
Due date: October 16 (40 weeks)
Since the period is only 4 days off when your real period was, count the January 23 date as accurate to within four days, i.e., a range of January 19 - 23). It has nothing to do with December, let alone December 17.
Don't worry about paternity.
Thanks for the response...although my period started on January 5th and lasted only 3 days, which is abnormal for me, I still am worrying over nothing? Because I don't remember the exact dates I had sex with my husband, but I know it was not during that Jan 19-27 range because I was working heavy hours that week.
Do you have an earlier ultrasound than the one in your 25th week?
Yes...I have had several..the earliest ones said I was 8 days further along than the original day. The last one I had, said I was 4 days further along...
This is my other problem...during that time my relationship was on again off again with my husband...I had sex with that guy one other time on January 21st I believe, but he did not ejaculate...Your response about conception date has me really worried.
I had a vaginal ultrasound on Feb 14th, and it said I was 5 wk, 1 day I believe. I had another on Mar 11, and it said I was 9 weeks, 2 days which was about 4 days ahead of the previous one.