Expert: Catherine Beier MS CBE - 8/9/2009

Question
QUESTION: I had intercourse with my partner December 19-29 2008. ( i had a very light spotting that i assumed was my period lasting from the 28-30. However is was not like a usual period at all) I then had intercourse with another partner January 13-16th. I am pregnant now and don't know who the father is. My due date is October 14th and my estimated date of conception is January 7th. However my due date was determined by ultrasound when i was already 18wks along. I had to do the ultrasound because/ i have irregular periods and no other date could be fully decided on because of the lightness of the December spotting.

I am really scared about this because the partner i was with in January was just a temporary fling during a mild separation from my long term boyfriend in December. I know that the dates are much closer to the January partner but i really want to know what the chances are of it being a December conception.

ANSWER: Ashley,


Based on your due date, you most likely conceived around January 21st, not the 7th.  January 7th would be the most likely date of your last period.


Your irregular cycles explain why the dates don't quite match up; you ovulated later in your cycle than is typical.



The chances of a December conception are slim to none.  Ultrasound at 18 weeks can be off by a week, which still means that the man in January is the father, since the dates of intercourse are more than 2 weeks apart.


Best,
Catherine

---------- FOLLOW-UP ----------

QUESTION: If i conceived around the 21st, how is it possible that i be a little over 30 weeks? I did not have a period on the 7th of January at all. Are you saying that's when it should have been if I were on a regular cycle? from the last time i had intercourse in December on the 29th, to the 7th of January is only 9 days difference. Your saying that it's still impossible to have conceived then? If i had conceived on the 21st wouldn't i be 28 weeks along? I guess I'm not sure how that works?

Answer
Ashley,


Yes, January 7th would have been when your last period would be expected if you had a regular cycle.


You are confused on how pregnancy dates are calculated.  Pregnancy due dates are calculated assuming every woman has a cycle of 28 days and ovulates on day 14.  It's calculated as 40 weeks from the date of your last period, not your conception date, which is typically two weeks after the start of your period.  This means that for the first two weeks of every pregnancy, you're not even pregnant yet.


So, although your last period started in December, you didn't ovulate and conceive until around January 21st.  Since you don't have regular periods, you would count back 40 weeks from your due date to arrive at the assumed first date of your last period, and you'd count back 38 weeks from your due date to find your conception date.  If you do so, you'll see you come up with the dates I gave you.


Again, your dates of intercourse are more than 2 weeks apart, not 9 days as you said previously. Sperm can only live in the reproductive tract for up to 5 days, meaning there's no way that the first man can be the father given your due date.



Best,
Catherine