V. cute about Eulalia's ds wanting to take the baby out to have a look !
Ref: Downs - apparently there is only a very tiny risk (1%) of Downs in a cousin impacting your chances. Your cousins may have been karyotyped (where their chromosomes are analysed) to check if they have any genetic characteristics which may impact their future risk profiles etc. etc. Perhaps you could ask them if they had this done? It becomes more relevant in cultures where cousins commonly intermarry. Sorry, but I don't really know much more about this, though I googled to this site which has a good summary of relevant info.
Bagpuss30 - I'm not a statistician (computer background), but did some stats as part of my psych degree. I find all the mathematical games relating to risk calculation for chromosomal abnormalities, due date calculation etc. very fun to explore. It's probably just a result of having too much time on my hands, being a healthy worrier, and being desparately excited too !
I've read several times that less than 5% of babies are born on their due date, so although it's very comforting to aim for 'goal', I wouldn't worry very much (particularly in 1st trimester) when your dates get shoved around. Perhaps pay more attention towards the middle / end as growth charts can highlight restricted / accelerated uterine growth which is something worth monitoring more closely.
If anyone is interested, the training manual for sonographers doing 11-13 weeks Dating / Nuchal scans, with full background on interpretting test results etc. Is linked to from this page. Incredibly long pdf document to bore the overly curious (like myself) !