When WW2 American veterans returned home in 1945, one of the well known effects was the demographic one known as "Baby Boom" - a sharp spike in birth rate from 1946 till 1957 or 1964, depending on your definitions.
Was there a similar demographic effect in USA (or elsewhere in the world) from returning veterans of WW1?
Going by the birth rate data, it would seem that there was little change in the birth rate between 1910 (30.1 per 1,000), 1915 (29.5 per 1,000) and 1920 (27.7 per 1,000).
It can be argued that the war didn't affect the population at all in that respect. However, another explanation can be that there are two conflicting forces working simultaneously. For instance, this was a period of increasing urbanization, which tends to lead to lower birth rates. This intersects with a baby boom after WWI, so the figures stay stable. Obviously, though, this is all just conjecture. I doubt it is possible to gather conclusive proof of peoples' motives.
Also, looking at the data for the 1920's, it seems that the "baby bust" between the wars can be correlated to the prohibition period. Birth rates start declining in 1920, have their lowest point in 1935 and are already rising by 1940, well before the American involvement in WWII started.
World War I did not produce a Baby Boom, because the so-called "Lost" generation that fought it was ALREADY a Baby bust generation. The war just exacerbated the effect of this cohort producing fewer children.
World War II had a LIBERATING effect on Americans. Women (included married women) had somewhat stopped sleeping with their husbands during the depressed 1930s.
When my mother came to the U.S. in the 1950s, she, and most of her neighbors talked about this a lot. They were referring to an older generation of women, and resolved NOT to be like this. Which is why they created a Baby Boom. The OLDEST of these neighbors tried to teach mother how to avoid her husband's (my father's) advances. But mother wasn't "buying."
But when the GIs returned in triumph as the world's most eligible bachelors, they married (and produced children) in record numbers. Because if American women wouldn't marry them, foreign "war brides" would.
...the maximum birth rates in the immediate post-war years were usually below the pre-war average and in practically all countries the long-term downward trend already in evidence before the war was resumed. A considerable part of this later decline reflects the reduction in the number of potential fathers -the young men who were killed during the war or who were so badly maimed that they did not marry.
French population growth (for the 40 years) from 1871 and 1911 was 8.6%, compared to 7.0% (for the 20 years) from 1919 to 1939. That is most definitely a baby boom. (Note that extrapolating from 20 years to 40 years, the post-war 7% translates to a 14.5% rate over 40 years.)
German population growth over the same two periods was respectively 59% and 15%, so there is definitely a baby bust in Germany.
However a baby bust in Germany was still a substantially greater growth rate than the contemporary baby boom in France, and Germany post-war was increasing in population at a much greater rate than Franc. This demographic knowledge was well known to both French and German politicians and planners of the inter-war years, and affected many decisions including the construction of the Maginot Line; and the respective decisions by French and German Generals to respectively imagine a defensive or offensive next war.
Here is a source with more details on European population growth during the inter war years
