No idea on LMP - best way to get an EDD???

As the title suggests, I have no idea on my LMP (slack I know :redface:). DH and I only DTD on July 18th and 19th - and my previous cycle was abnormally light. Cycle lengths regulated to 33 days for about the previous 8 cycles, however the last two (where I have no idea on dates) seemed to have longer cycle lengths, albeit by 5 or 6 days.

First blood test on July 27th - HCG was 226. Dr said 'the president had an expected gestation of 2-3 wks'.

Second blood test on August 2nd - HCG was 2256. Dr said 'the president had an expected gestation of 4-5 wks'.

After reading Arimeh's Eden thread about the accuracy of dating scans, I'm concerned that all I can really go off of for an EDD is yep, a dating scan :rolleyes:.

And I just query how the scan 'aligns' with blood test results? There was only 6 days between blood test one and two, yet a larger increase in the expected gestation.

Bah! I'm confused!!! HELP!

a dating scan before 12 weeks(under 10 i think is best) should give you a reasonably accurate EDD, but even if your not happy with the date be firm and choose one you are comfortable with :)

There are so many things which affect EDD's! With both of my last 2 pg's I got a couple of EDD's even though I knew when my LMP was for sure. With DD's pg I was given about 3 EDD's spanning about 1 1/2 weeks apart. With this pg, I know I got my LMP between 10th and 19th April though I am not sure the exact date of it starting. I had an u/s at what I estimated to be about 7 weeks. This scan said the pg was 7w4d. OK, that would be about right maybe a bit more than I though I could be, but hey what is a few days? My nuchal trasluscency scan (supposed to be done between 11-14weeks apparently) I had when I was just 11w2days according to dating scan. This u/s said baby measured 12w1d. A whole week out, quite a fair bit out from what I was guessing when my LMP was due. So either baby has had a huge growth spurt in the past few weeks or I was waaayyy out with my guestimations. I am assuming that my morphology scan will be telling me that baby is measuring even bigger than before. I tend to have fairly big babes. And long ones too. I'm saying my EDDis about the 17th of Jan. My first scan puts my EDD at around the 21st Jan. A week either way I'm not too worried. I will just be aware of baby in the weeks leading up to my EDD.
They say the earlier you can have a scan the better, I think about 6-7weeks is the best time.

Dating scans are actually pretty accurate if done early. Ds was born on his dating scan due date and DD was born 2 days after hers.