baby boom 
noun informal
A temporary marked increase in the birth rate, especially the one following the Second World War: the number of college graduates mushroomed in the early- to mid-1970s, thanks to the baby boom [as modifier]: the baby boom generation...
- He observes that the post war baby boom occurred between about 1948 and 1965.
- There has been a considerable geographical shift in population over the last fifty years, disguised in part by the overall increase resulting from the baby boom.
- Following the baby boom, fertility rates declined over the late 1960s and 1970s.