The aim of this study was to determine the elements of fatigue in occupations which constitute possible risk factors for the course of a pregnancy, and, in particular, that could cause premature birth. In 1977-1978, a total of 3437 women in France were surveyed after giving birth in two maternity hospitals. One hospital was located in Lyon (a large city) and the other in Haguenau (a small town). Among 1928 working women, it was found that certain occupational categories are more prone to risk of prematurity than others. The authors carried out an analytical breakdown of the job into its diverse components which led them to define five sources of fatigue and to construct an index capable of detecting the strenuous working conditions. There is a significant relationship between the prematurity rate and the number of high fatigue scores observed in the job. When the number of scores varies from 0 to 5 the rate of premature births increases from 2.3% to 11.1%. This relationship remains significant after controlling for confounding factors.