http://en.wikipedia.org/wiki/Demographics_of_France#1800_to_20th_century said:
Demographics of France
1800 to 20th century
Starting around 1800, the historical evolution of the population in France has been extremely atypical in the Western World. Unlike the rest of Europe, France did not experience a strong population growth in the 19th century and first half of the 20th century. The birth rate in France diminished much earlier than in the rest of Europe. Consequently, population growth was quite slow in the 19th century, and the nadir was reached in the first half of the 20th century when France, surrounded by the rapidly growing populations of Germany and the United Kingdom, experienced virtually zero growth. This, and the bloody losses in France's population due to the First World War, may explain the sudden collapse of France in 1940 during the Second World War. France was often perceived as a country facing irrecoverable decline. At the time, racist theories were quite popular, and the dramatic demographic decline of France was often attributed (particularly in Nazi Germany, and also in some conservative circles in England and elsewhere) to the genetic characteristics of the "French race", a race destined to fail in the face of the Germanic and Anglo-Saxon "races". In addition, the slow growth of France's population in the 19th century was reflected in the country's very low emigration rate. While millions of people from all other parts of Europe moved to the Americas, few French did so. Most people in the United States of French extraction are descended from immigrants from French Canada, whose population was rapidly growing at this time.
To understand the demographic decline of France, it should be noted that France was historically the largest nation of Europe. During the 17th century one fifth of Europe’s population was French (and more than one quarter during the Middle Ages). Between 1815 and 2000, if the population of France had grown at the same rate as the population of Germany during the same time period, France's population would be 110 million today -- and this does not take into account the fact that a large chunk of Germany's population growth was siphoned off by emigration to the Americas. If France's population had grown at the same rate as England and Wales (whose rate was also siphoned off by emigration to the Americas, Australia and New Zealand), France's population could be anywhere up to 150 million today. And if we start the comparison at the time of King Louis XIV (the Sun King), then France would in fact have the same population as the United States. While France had been very powerful in Europe at the time of Louis XIV or Napoleon, the demographic decline the country experienced after 1800 helped it to lose this advantage.
After World War II
After 1947 however, France suddenly underwent a demographic recovery. In the 1930s the French government, alarmed by the decline of France's population, had passed laws to boost the birth rate, giving state benefits to families with children. Nonetheless, no one can quite satisfactorily explain this sudden and unexpected recovery in the demography of France, which was often portrayed as a "miracle" inside France. This demographic recovery was again atypical in the Western World, in the sense that although the rest of the Western World experienced a baby boom immediately after the war, the baby boom in France was much stronger, and above all it lasted longer than in most other countries of the Western World (the United States being one of the few exceptions).