I was wondering why doesn't my LMP and due date match? I had my period in the beginning of December(2014). And I had sex on the 22nd of December and then 3rd of Jan. I was wondering if there's any way of knowing which date I conceived. The only thing is that I don't know when I ovulate. That's why its hard for me and my cycle is regular always comes on time. The only difference was that the month of December it came a couple days early.
Every time I count back 23 weeks from this week (6/2/15) it lands on the week of December 22nd, but when I put my due date on websites it always tells me wrong dates, especially LMP.

What due date do you have? Are you considered to be 23 weeks today? Did you have an early ultrasound? Have you had more than one scan?

Often due dates are given from the last period. However, rarely are you actually pregnant from that time. The due date may be altered a bit after an early scan when fetal measurements can be taken.

Your due date is a bit of a guide... few babies are born on their due date and gestation can vary a bit from woman to woman. Take that, along with just a rough idea of conception ( in most cases), and you can see why you have several weeks at the end of pregnancy in which it would be considered normal to give birth.

A couple of weeks either side of your due date can be normal. It's not usually a concern unless a paternity question comes into play.

I've had 2 ultrasounds so far. My recent one my doctor gave me due date of Sept 25 according to measurements. Yes I'm 23 weeks and 4days today. My first ultrasound I was 9 weeks and 6 days and the lady gave me a due date of sept 28 but that was my very first one to confirm my pregnancy. My due dates that where given to me aren't based off my last period. So then when I go into labor that day I give birth and count back from that week I give birth is most likely when I give birth, paternity test is my last option because the father refuses.

Ok, there is no way, absolutely no way, to know which date you conceived on. Babies come early, they come late, there are almost never, on the exact date.

Conception dates are give or take a week, sometimes two. So no, with the dates of sex, so close, the only way. And I mean only way, to know for sure, will be a DNA test after the baby is born.

I assume paternity test, since you say (father refuses) then get the other man to do a DNA test? If not, merely go to court and get a court order, to force them to take the paternity test.

Your confusion over the due date is because you don't know who the father is? You won't solve that mystery until the child is born.

As was mentioned... you WILL need to do a DNA test after the child is born if the reason you are so concerned about when you actually conceived is because more than one man could be the father. It could easily be up to 2 weeks before or after their best guess as to when it happened. That's a 4 week period of uncertainty.