BACKGROUND The early research on the baby boom tried to account for it as a logical recovery following the end of the Second World War (WWII). But it cannot be understood merely as a post-war phenomenon because its origins go back to the 1930s and early 1940s. OBJECTIVE I shall describe the methodology used to measure the total and marital baby boom and provide a detailed description of it. I shall attempt to explain the possible reasons that led to the sharp increase in the marital fertility rates and its subsequent decline. METHODS I will use various fertility indices that track the historical development of fertility (total and marital; period and cohort). RESULTS I show that there are major differences in the measurement of the baby boom depending on the index used. I found that the baby boom is highly heterogeneous in the 25 countries that form part of my study. It represented the logical response that families made to one period of prolonged political, economic, and military crisis (the crash of 1929 and WWII). CONCLUSIONS Researchers who use only the total fertility indices are really analysing only the nuptiality boom, which took place during those years, rather than changes in reproductive behaviour.