As Holly noted in her guest post on my blog, married women, in the 17th and 18th centuries, would become pregnant on average five or six times. This is far less than “natural” fecundity. So what was happening?
For one thing, people married relatively late. In 1789 France, the average age of first-time couples was 26.5 for brides and 28.5 for grooms. True, nobles married much earlier, generally as teenagers, but they represented only about 1% of the population, a tiny minority. For most French people, at least 10 years of reproductive life were thus “lost” to late marriages. The tremendous social stigma attached to out-of-wedlock births made them accidents to be avoided at all costs.
Once women married and gave birth to children, the most widely available birth control technique was breastfeeding. Peasant women in particular nursed their children--and served as wetnurse to others--well into toddlerhood, which allowed them to space out their pregnancies. Rousseau, for reasons independent from contraception, strongly advocated breastfeeding in his very influential Emile (published 1761) and the practice soon became fashionable among upper-class women as well.
Even the illiterate knew empirically about the contraceptive effect of breastfeeding. Other techniques, however, required an advanced understanding of conception. This was reserved to the more educated segments of the population. Pornographic novels (a thriving genre in pre-revolutionary France) extolled the virtues of the withdrawal method, and barrier devices like sealskin condoms and sponges dipped in an acidic liquid such as vinegar.