The Second World War did not have such an effect. At the time, human population was in the neighborhood of 3 billion, and growth was accelerating globally. It passed about 2 billion in 1925 and 4 billion around 1980. In the context of such a large number of people, the loss of several tens of millions of lives--even in the short span of six years--was swamped by both the number of births around the world and the increasing life expectancy. After all, the time to double from 2 billion to 4 billion was just fifty years, easily enough to absorb the heavy casualties of the Second World War. Viewed as a graph, it makes hardly a dent (that chart's from Wiki, but it uses US government info that is easily verifiable). I got my info from the rather old but I hope still fairly reliable Atlas of World Population History, by McEvedy and Jones (Penguin, 1978).

Whether this has ever happened as a result of war is hard to say. I suppose it's possible though, I'm not sure how WWII compares to previous wars in terms of proportion killed. And, there have been times in human history when population was stable or shrinking, so at one of those times the question doesn't really work.