During the baby boom, more French women bore children than ever before, just as in United States. Raising children at a young age became the norm and expectation. During the boom years--from about 1945-1965--fertility increased for all women younger than 35. Age at first childbirth decreased to a low for women born in the 1940s, whose age at first intercourse declined without the introduction of medical forms of contraception. About 90 per cent of all French women who came of age after 1945 had children; this trend has continued to the present day.
Was the baby boom a sudden change? Was it really a boom? The fertility rate, what demographers call the Total Fertility Rate for a specific period, dropped sharply during the war years and then spiked even more dramatically in the late 1940s, to three children per woman for four years running. So there is no mistaking the boom in terms of the numbers of babies born after the war, more than making up for the fertility decline during the dark years of WWII.
But if we look at a different fertility measure, the cohort completed fertility, the boom looks less dramatic. Cohort completed fertility is the number of live births for the cohort of women over their lifetimes, so it smooths out the impact of events, even those as momentous as the Second World War. It turns out that women who came of age during the baby boom fit into a long-term trend of rising fertility that had begun a generation earlier. From a low point for the cohort of women born in the 1890s (who bore an average of 2.0 children in the 1910s and 1920s), the rate rose steadily, sweeping the baby boom mothers along, until reaching a high of just over 2.6 children for women born around 1930. Increasing fertility was a long-term trend in the middle years of the twentieth century of which the baby boom was a part.
In fact, in France the post-World War II baby boom was not as dramatic as the boom after the first world war. More significant was the sustained economic growth undergirding the boom.
Source: Laurent Toulemon, Ariane Pailhé, Clémentine Rossier, "France: High and Stable Fertility," Demographic Research 19, article 16 (July 2008): 503-556. Image is Figure 1, 508.