If you are irregular, you should NEVER go by your LMP. I always go by my ovulation date (I chart). I suppose U/S would be accurate too, but I've never had a dating scan.
When i 1st found out i was pregnant they thought i was 6 weeks due to my lmp but my blood level wasnt matchin n they couldnt c anything so they thought i was having a ectopic preganacy come to find out i was only 3 weeks i had ovulated late now im 26 weeks with a healthy baby girl...it takes time dont worry or stress your self your goin to have a healthy baby