I don't understand, can doctors be wrong about conception dates and due dates?

Well its all just a gues.. there is no one who can tell you exactly when you conceived or when you are due.. babies come early and late all the time... and you can conceive a couple days after sex... so ye, it is very possible... but judging by the LMP and ultrasound technology, doctor are able to give a fairly accurate guess...

The doctors are guessing using info that you supply them. So it is not a mistake, since it was only an educated guess at best.

The only way to be 100 is to only to have sex once during a large time frame around that time. But then unless it is a multiple partner issue, why does it really matter he exact date

As a new nurse in the Maternity ward, I can tell you that conception dates are totally a guessing game, unless of course the doctor is in the bedroom with you. LOL

What we use is something called Nagele's Rule. It takes the first day of your last period, then subtracts 3 months, adds 7 days, add 1 year, and you have a possible due date. This is still all a guess as there is no way to tell EXACTLY which day you ovulated.

When you ovulate, your egg is only viable for conception for 24 hours, yet sperm can live in a woman's body for up to 3 days.