The proportion of twin births stood at 15 per 1,000 in France in the early eighteenth century and returned to this level in 2000 after falling substantially in the intervening period. It has risen by 70% since the early 1970s under the dual influence of infertility treatments, which explain two-thirds of the rise, and increased age of childbearing, which accounts for the other third. After analysing the variations in twinning rate in France over time, the article examines the various contributing factors and focuses on two in particular: voluntary birth control and selection by fecundity. Women who have produced twins less frequently undertake additional pregnancies than women who have had a single birth. The consequences of a twin pregnancy on the probability of additional childbearing are measured by analysing the histories of almost one million French women recorded in successive family surveys. Lastly, the article examines the twinning peak recorded in France during the First World War and just afterwards, in 1919. This peak can be attributed to an effect of selection of the most fecund couples, who also have a higher propensity to produce twins.