Did you consider asking the doctors from Infection Control what the rules/recommendations were regarding a pregnant woman caring for a vented pt with TB? I'm a pregnant nurse, working in the ED, who has been around pt's with TB, sometimes in isolation and sometimes not....that said, I've also been told that the vent is a closed system and the threat of catching TB is not much to worry about.
In these cases what is "believed" may not be actual fact and the best thing would be to ask those who can provide information to either reduce or increase (!) a staff person's concern.
That said, you can never escape that one incident where "something" happens, so allowing a pregnant nurse to care for another pt while taking over that person with TB is very much appreciated.