I did a little digging around last night and found (among much else) this paragraph, based on work done by Peter Laslett, a very respected scholar:
The rate of extramarital births during the sixteenth century is generally perceived to be quite high, but it later sank during the age of absolutism. It is stipulated that only 2 to 3 percent of all births in the mid-1700s were extramarital, but a century later numbers hovered between 7 and 11 percent in the Nordic countries and around 7 percent in France and England. Certain countries and regions had higher figures; in Iceland more than 14 percent of all births occurred outside of marriage, and in the Basque Country the illegitimacy rate was exceptionally high. The following century or so, from the 1840s to 1960, witnessed a new decline of illegitimate births, particularly conspicuous around the turn of the century. Regional differences, however, were still to be found.