You clearly wouldn't need preventive withdrawal if the workplace was wholly safe, but at the moment, most of the occupational exposure limits and regulations in Canada have been based on the work of men and the susceptibilities of men rather than women. One of the major issues is to make sure the recommendations about workplaces take account of the fact that increasing numbers of people in trades are women, and that they may be affected differently, pregnant or not, from men by workplace exposures.
On April 4th, 2017. See this statement in context.