The historical demographic studies on fertility have largely focused on the early stages of the transition and few of them have specifically examined the interwar period. Yet fertility then reached very low levels, often below the generation replacement level. Two main hypotheses are generally advanced to explain these low levels of fertility. This would be either the extent of the decline started decades earlier, either the adaptation of the behaviors to economic, political and social changes of the time and the deterioration of living conditions resulting. 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 if they verify (or not) the explanations.