My belief is that it's more to do with the strenuous nature of the trade. We are currently looking at the effect of the welding fume on the outcome of a pregnancy. I don't have the results, but I would be surprised if the welding fume itself caused miscarriage, whereas there's good evidence from the Montreal study and others that in the first trimester of pregnancy, strenuous work can lead to miscarriage.
On April 4th, 2017. See this statement in context.