That's true. It can vary on the pregnancy itself. It can vary on the woman's history of previous miscarriages, for example.
I haven't said, but perhaps you'll allow me to now, that it's important also, if we are talking about preventive reassignment, that the woman's choice comes in. The employer shouldn't be allowed to insist that she take a reassignment. It is for the woman herself to make that decision, in discussion with her doctor, obviously.