Question is why was this the case? I feel a natural explanation is it's a reaction to there being less people around on account of the huge number of deaths, in

Question is why was this the case? I feel a natural explanation is it's a reaction to there being less people around on account of the huge number of deaths, in which case there's probably little reason to worry: if birth rate increases as population drops then the system should tend to equalise itself. If it's something else related to a gigantic war specifically then it's a bit less certain (one very important question is was the growth just a displacement in time from all the couples who could not have a child during the war but now can?).