New to the series so sorry if this is a dumb question but Bastardy only exists from the moment of physical birth, not conception? Like if a woman is nine months pregnant, suddenly gets married and gives birth the next day, that baby is legitimate right?

Sort of? Basically what happens is that you get a social fiction that the obviously pregnant bride is nothing of the sort and everyone pretends when she gives birth a few months later that the birth is premature despite the surprising heft of the child. 

But that’s not quite the same as the custom being that birth not conception matters. The social fiction is there to preserve the custom even in the breach.