A typical war has a large impact on demographics. People die, mostly men. Fewer people are born, because men a missing, both because they are on the battlefield or, as mentioned, dead. At the end of the war, fertility shoots up to catch up for "missed opportunities", and we get baby booms. Well that is the conventional wisdom, and it is not necessarily right. For example, there is some evidence, discussed here before, that the baby boom after World War II was not about the men returning home and catching up on baby making. And one could also challenge the idea that fertility drops during the war because of the absence of men.
Guillaume Vandenbroucke does this for World War I in France. he draws a model of fertility choice where couples factor in that the potential father may die in war. Of course, this reduces fertility, but the question is how much. To get an answer, the model is carefully calibrated to pre-war fertility, mortality and income figures. Then 97% of the drop in fertility is explained by expectations. Of course, this assumes that the French correctly predicted the probability of death. Given that this war lasted much longer than expected and introduced killing technologies of never-seen-before efficacy, I doubt this is a correct assessment of the expectations at the time.