Weird dates-can anyone help?

[ASmidgeofMidge]

I have what seems like a slightly weird situation with my dates, and am giving myself the fear Im going to have a giant baby! Am hoping someone can help.
LMP was 23rd April, and felt like a 'normal' period iykwim. DH and I were using (Amazon cheapie) opk's and dtd 7,8,9 May - quite a big hiatus either side due to his night shifts. POAS w/c 20th May (I know-was v impatient) and 'twas +ve.
Had 12w scan on 15th July and they said I was 12w 5 days so am now about 14w... But my calculations suggest I can only be 11 at the most??!

Could I have conceived in April and had a random period? I did do some poas-ing in April with no +ve result....

Hmmm...

They count how far along you are by reference to your LMP. On dates, you'd now be 13+4.

And although medical folks seem to think scans are completely accurate, they deal with nature so there must be some variation. On dates you'd have been 11+6 at your scan so if you ovd a bit earlier and implantation was a bit sooner than average that could make you a bit further along.