What does it mean if ultrasound, doesn't match lmp, in terms of conception date and due date?

Depends: It either means that conception did not occur according to your last menstrual cycle or unfortunately the pregnancy may not possibly be normal.

Ultrasound dating: It depends on the timing of ultrasound. First trimester ultrasound is accurate within 5-7 days. 2nd trimester ultrasound has accuracy of +/_ 2 weeks. Lmp dating also assumes a 28 day cycle. There is no test that can date conception more accurately than a 1 week window. Due date is based on LMP unless us off by more than 1 week 1st trimester or 2 weeks in 2nd.

Depends: If you are certain about your LMP date and you have regular periods, dating pregnancy by LMP is accurate. If not, rely on ultrasound (us) dating for due date. Us done at 6-7 weeks is most accurate (to about 5 days)but is still accurate to about 10 days at 18-22 weeks. Us gets less accurate the later it is performed in pregnancy.