Hello, my due date is December 20th and I must know the date I conceived...

There is really no way to tell without room for error. The date of you last menstral period will help pinpoint it as will knowing how regular your cycle is.

It is very difficult to say. You can conceive a child anytime during your cycle. That leaves a 28-day window for most women. I can tell you that 12-14 days after the first day of your last period is when women are most fertile (on average) but, that doesn't mean it is hen you conceived. Not to mention that sperm can live inside you for several days, so you could have sex one day and actually conceive 6 days later... If you are trying to figure out who the father is, I'm afraid you will have to wait until birth. Good luck.

Your doctor will have the possible date of conception in your file. You can ask him/her. With my last pregnancy I asked out of curiosity and my doc gave me the EXACT date I was thinking. So, why not ask your doc at your next appointment.