Okay. When you have a scan, they give you the number of weeks pg you are as from two weeks before conception, just like if you calculate from LMP. Otherwise it'd be really confusing and you'd keep having to add weeks and stuff. So really they're saying you conceived around the beginning of January and acting as if your LMP was the 18th Dec. But don't worry if that's impossible - it's hard to tell at this stage and babies grow at different rates, so chances are they'll adjust back again. So yes, they're adding 40 weeks - the full gestation - to the 18/12/08. It is really confusing I know, but they do it like that to try and make it less complicated lol - go figure.