OMG - just watched a Time Team thingy about War of the Roses, and Baldrick has just done a big thing about where the Duchess of York was while her husband was fighting wars, and then attempted to decide if Edward IV was legitimate or not. They did this by dating the conception of Edward 40 weeks before his birth.
If the entire historical community, the BBC and Baldrick himself can't understand that pregnancy lasts 38, but is dated by 40 weeks from the previous period as the most reliable date, then what hope do we have for women's bodies EVER to be understood?
I'm sure that they know really, but shifting the date to the correct one makes Edward's legitimacy far more likely, and pretty much wipes out the rest of the program, so hey, ho. Let's ignore facts and spread complete piffle about pregnancy just for the sake of a bit of TV!