A web page on welding fumes and health that you might want to look at is: http://www.gov.ab.ca/lab/facts/ohs/ch032.html It's from Alberta Human Resources and Employment.
Pretty much any fumes generated by welding and cutting we all should avoid breathing, pregnant or not. Something else that is often overlooked is ozone. Ozone is generated in the weld environment by UV light acting on oxygen molecules and is at the worst levels when using argon in welding and nitrogen in plasma cutting because of the light wavelengths generated by electric arcs in those gases. There is no mask that will filter ozone out, all you can do is provide ventilation (or air supplied respirators). Tests our safety people have done in our shop indicates that the farther from the arc you get the lower the ozone levels (pretty common sense).

Something that really concerned our medical consultants was the elevated levels of copper in fumes coming from GMAW using copper coated wire. (L-56?)

The big part of all this is that no one can tell you how much is too much, either ozone or welding fumes, although there are some guidelines from OSHA and such. The comment about not meeting (m)any 70 year old welders still welding is something to think about.

As a minimum you may want to increase the ventilation around the pregnant welder.

CHGuilford