Here in the U.S., the FAA doesn't have any regulations addressing this sort of thing. It's pretty much up to company policy and I know that policies differ greatly from carrier to carrier.
As a Captain for many years on two seperate airlines here, I had a couple of encounters with the issue. In one case, a FA (Flight Attendant as they are termed here) reported for work and she muct have been 7 or 8 months pregnant; either that or she was just really big. I had issues and called our chief pilot with my concerns. Those concerns centered on not only her health, but in an emergency, could she accomplish all that was needed for the safety of my passengers. The chief pilot and VP of in flight came to the gate and after interviewing the young lady, she was taken off of active inflight. She remained on payroll assisting with time cards, etc. I thought she would probably be upset with me, but later she said she was happy that I had intervened. So... it's up to the employee and her company since the governing agency has no input, other than for saftey...