Baby Boomer
Well, what do you think all those horny soldiers did the minute they got home from WW2, The Big One? Yeah. Giggity.
It was a boom that followed Hiroshima's pinpoint ending. What followed was a boom in babies and a subsequent draw on resources to expand schools, parks, and overall country infrastructure to accommodate a fast growing population. The boom lasted until the early 1960s, when that boom died and the immigration boom picked up the slack, more or less.