baby boom

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A sudden, large, sustained increase in the birthrate, especially the one from the later 1940s through the early 1960s.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun singular only The increase in the birth rate following the return of servicemen at the end of World War II.
• noun Any similar increase in the birth rate.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the larger than expected generation in United States born shortly after World War II