Same demographer friend (more accurately, ex-girlfriend who was studying social and economic history at the time) told me that illegitimacy varied a lot by region in the early modern period. If I recall correctly, there were Northern rural communities where the first child was typically born before marriage. Or maybe so soon after that the parents must have known the women would bear a child. This was because marriage was seen as marking when you set up house, rather than the start of sex, and because you wouldn't fix a relationship until fertility/combatibility was clear. People may have become engaged and pledged to each other first, mind.