The end of World War II brought a baby boom to many countries, especially Western ones. There is some disagreement as to the precise beginning and ending dates of the post-war baby boom, but it is most often agreed to have begun in the years immediately after the war, though some place it earlier at the increase of births in 1941–1943. The boom started to decline as birth rates in the United States started to decline in 1958, though the boom would only grind to a halt 3 years later in 1961, 20 years after it began. In countries that had suffered heavy war damage, displacement of people, and post-war economic hardship (e.g., Germany and Poland ), the boom began some years later.
In the United States
Most Baby boomers are now in their 60s or 70s, with the oldest member of the boom being around 76. The trailing boomers are in their late 50s, with the youngest being 57. In the economy, many are now retiring and leaving the labor force .
In 1946, live births in the U.S. surged from 222,721 in January to 339,499 in October. By the end of the 1940s, about 32 million babies had been born, compared with 24 million in the 1930s. In 1954, annual births first topped four million and did not drop below that figure until 1965, when four out of ten Americans were under the age of 20.
In the years after the war,; the mood was now optimistic. During the war, unemployment ended and the economy greatly expanded; afterwards the country experienced vigorous economic growth until the 1970s.
Definition of the boom years
It is important to distinguish between the demographic boom in births, and the actual generations born during that period.
As shown in the birth rate chart to the right, the baby boom in the United States is as much defined by the low birth rates that preceded and followed it as it is by an exceptionally high post-war fertility rate. The birth rate from 1946 to 1964 is much lower than the birth rate prior to World War I, although it is high in comparison to the time periods immediately preceding and following 1946–1964.
The exact beginning and end of the baby boom can be debated. The U.S. Census Bureau defines baby boomers as those born between mid-1946 and mid-1964,  although the U.S. birth rate began to shoot up in 1941 and to decline after 1957. Deborah Carr considers baby boomers to be those born between 1944 and 1959,  while Strauss and Howe place the beginning of the baby boom in 1943. The following table shows changes in U.S. population during the period of U.S. involvement in World War II and for the five years thereafter, based on U.S. census information. 
|Year|| US resident population |
| Net change |
| Percent |
Historical and social background
When the war ended in 1945, millions of veterans returned home and reintegrated into civilian life. To help this process, Congress passed the G.I. Bill of Rights . This encouraged home ownership and investment in higher education through the distribution to veterans of loans at low or zero interest rates.
Returning veterans married, started families, pursued higher education, and bought their first homes. With veterans' benefits, the twenty-somethings found new homes in planned communities on the outskirts of American cities. This group, whose formative years covered the Great Depression, was a generation hardened by poverty and deprived of the security of a home or job. Now thriving on the American Dream, life was simple, jobs were plentiful, and a record number of babies were born.
Many Americans believed that lack of post-war government spending would send the United States back into depression. However, consumer demand fueled economic growth. The baby boom triggered a housing boom, a consumption boom and a boom in the labor force. Between 1950 and 1960, the nation's real GDP rose almost 50%, from $2.2 trillion to $3.1 trillion (in 2009 dollars). The middle class grew and the majority of America's labor force held white-collar jobs. This increase led to urbanization and increased the demand for ownership of cars and other 1950s and 1960s inventions.
Marriage rates rose sharply in the 1940s and reached all-time highs. After World War II, Americans began to marry at a younger age: the average age of a person at their first marriage dropped to 22.5 years for males and 20.1 for females, down from 24.3 for males and 21.5 for females in 1940.  Getting married immediately after high school was becoming commonplace and women were increasingly under tremendous pressure to marry by the age of 20. The stereotype developed that women were going to college to earn their M.R.S. ( Mrs. ) degree.
Family size increased sharply throughout the baby boom: the average woman bore 3.09 children in 1950 which increased to 3.65 children per family in 1960; the peak was in 1957, when the figure stood at 3.77. Most women became pregnant with their first child within seven months of their wedding; between 1940 and 1960, the number of families with three children doubled and the number of families having a fourth child quadrupled.
In the United States more babies were born during the seven years after 1948 than in the previous 30, causing a shortage of teenage babysitters . Madison, New Jersey , for example, only had 50 high-school girls to babysit for a town of 8,000, and any sitter could have had two sitting jobs at once if desired. $5 of the $7 that a California couple spent to go to the movies in 1950 went to the babysitter. 
Economist and demographer Richard Easterlin in his "Twentieth Century American Population Growth" (2000), explains the growth pattern of American population in the 20th century by examining the fertility rate fluctuations and the decreasing mortality rate. Easterlin attempts to prove the cause of the baby boom and baby bust by the "relative income" theory, despite the various other theories that these events have been attributed to. The "relative income" theory suggests that couples choose to have children based on a couple's ratio of potential earning power and the desire to obtain material objects. This ratio depends on the economic stability of the country and how people are raised to value material objects. The "relative income" theory explains the baby boom by suggesting that the late 1940s and the 1950s brought low desires to have material objects, because of the Great Depression and World War II, as well as plentiful job opportunities (being a post-war period). These two factors gave rise to a high relative income, which encouraged high fertility. Following this period, the next generation had a greater desire for material objects, however an economic slowdown in the United States made jobs harder to acquire. This resulted in lower fertility rates causing the Baby Bust.
In Canada , the baby boom is usually defined as occurring from 1947 to 1966, with over 400,000 babies born yearly. Canadian soldiers were repatriated later than American servicemen, and Canada's birthrate did not start to rise until 1947. Most Canadian demographers prefer to use the later date of 1966 as the boom's end year in that country. The later end than the US (baby-boom generation: babies born from 1946 to 1964; see preceding paragraphs) is ascribed to a later adoption of birth control pills.
The author and columnist Bernard Salt places the Australian baby boom between 1946 and 1961.
In the United Kingdom
After a short baby boom immediately after the war peaking in 1946, the United Kingdom experienced a second baby boom during the 1960s, with a peak in births in 1964, and a third, smaller boom peaking in 1990. The three peaks can clearly be seen in the UK Population Pyramid .  
European and South-Pacific trends
Many European countries, Australia and New Zealand also experienced a baby boom. In some cases the total fertility rate almost doubled. The American birth model, conceived by demographer Frank Notestein , was punctuated by an end to the upsurge in births and a return to pre-war levels. In many European countries the first year of the Post World War II baby boom was the year 1946, but in Germany the first year was the year 1955 in Finland the largest birth rate was in August and September 1945. Japan had two separate baby booms. The first started in 1947, the second in 1971. Prior to World War II, mortality rates in Europe and America were on a general decline due to improved nutrition and medicine, and a surge in births were previously not experienced at such a large scale. Based on this model, baby boom years for other countries regarded for having a baby boom are as follows:
- France 1946–1974
- United Kingdom 1946–1974
- Finland 1945–1950
- Germany 1955–1967
- Sweden 1946–1952
- Denmark 1946–1950
- Netherlands 1946–1972
- Ireland 1946–1982
- Hungary 1946–1957
- Iceland 1946–1969
- New Zealand 1946–1961
- Australia 1946–1961
- Japan 1947–1949, 1971–1974
In some of these examples, an "echo boom" followed some time after as the offspring of the initial boom gave rise to a second increase, with a baby "bust" in between. The birth years of the baby boom as noted being both short and long lived, creates what many believe to be a myth to the notion of defining baby boomers as one "generation", as a unified concept is clearly not possible. Indeed, multiple generations may be present in a single country such as Ireland where the boom lasted 36 years. This overlapping effect of generations is not illuminated when considering crude fertility rates. The only common ground for the collective boom is the same approximate starting year. This example can be applied to each state in the United States on an individual basis. The states with a census in place in 1946 saw fertility rates drop to pre-war levels throughout the 1960s, with the average being in 1964.