I went through a break up around the time I got pregnant, the break up happened December 13th so I am slightly concerned about my conception dates. I would like to know if it was the boyfriend I had or my partner after him. Most things I look up online say that you can find the day you conceived according to your last period. Unfortunately, I don't remember my last period. I was given the due date of September 16th after getting an ultra sound, can I use that to tell what day I conceived?

Why didn't you ask the ultrasound tech then?

I would possibly to work Naegele's rule backwards.
Example:
LMP = 8 May 2009

+1 year = 8 May 2010
-3 months = 8 February 2010
+7 days = 15 February 2010
In this case the person's expected due date is feb 15, 2010. You have a due date.. so subtract 7 days add 3 months and subtract 1 year and you should get the "supposed" last menstrual period. It depends largely on the time between having sex with your partners. Are we talking weeks or days? If its days, its goingt be darn hard to determine the who the father is. Good luck.

I wasn't told my due date according to my ultra sound the day of, it was mailed to me & now I am very confused. My ex and I broke up December 13th and I immediately went back to my high school sweetheart. I am trying to figure out if it happened before or after the 13th of December.

NO, I am sorry, you can come up with a date and it is close, give or take two or three weeks.

So as long you at leasted waited a few weeks to have sex between men, you may be able to narrow it.

But most likely no, you will have to get a DNA test after the child is born

I agree with Chuck.

The due date is just a guess, only 10% of babies are actually born on their due date.

Unless you know the exact day you ovulated, there's very little chance of you pinpointing the day you conceived.

Your only option is a DNA test after the baby is born.