From what I know, yes, mercury in anyone would cause harm. But it’s a double whammy if there are significant levels of mercury in women who are now pregnant or one day will bear children.
Just as using tobacco would be harmful to everyone, it is even worse for childbearing women since the unborn child will be developing within that chemical filled environment. What is a small level of toxin for a grown adult would be much more toxic, even deadly to a child, even more deadly to an infant, and yet even more so to an unborn child.
So, as I said, from what I know we should prevent everyone from getting toxic metals in their blood streams, but that is a big task. So to start somewhere, we try to stop the metals from spreading down into a new generation.