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The historical demographic studies on fertility have largely focused on the early stages of the transition, with few of them specifically examining the interwar period. Yet fertility at that time reached very low levels, often below the generation replacement level. Two main hypotheses are generally advanced to explain these low levels of fertility. Either the decline started decades earlier, or it was caused by the adaptation of behaviors to the economic, political and social changes of the time, and the deterioration of living conditions that resulted. This paper will first focus on the main explanatory theories of low fertility. The indicators used in empirical studies will then be examined to see whether or not they verify the explanations.