i wondered on this theory as well Spice - it makes sense that the embie can only release so much HCG which would get diluted as the blood volume for a larger lass is obviously going to be higher than for a skinny girl. there are so many other factors that can alter this though! i am a larger girl, and have always gotten a positive no later than 13 dpo, this time as early as 10dpo - it was pale, but definitely there! things like the sensitivity of the test obviously come into it (i don't have a lot of faith in some of the "super" sensitive ones being as sensitive as they say), how concentrated your pee is (if you are dehydrated, you'll get a stronger result) - and then other things like how many days after ovulation/conception implantation happened (it can be anything up to 12dpo). whether you conceived on o day or in the day afterwards etc...

i was at what should have been 15dpo when i had my BT for this cycle - and got a result of only 26 for hcg - at scans it came back showing i may have ovulated up to five days late (go figure - it was an IUI cycle - i thought ovulation was guaranteed at time of trigger!). i think it was more about how concentrated the urine was (i was feeling sick already and drinking lots wasn't working - also had mild OHSS so ovaries leaking water into my abdo instead of it being processed through kidneys!).

the ranges of "normal" hcg that early in pregnancy are really broad, and the "average" HPT only picks up the results for the "average" or above average HCG counts at that time...