Unless your doctor has specifically notified your company that you are not to work more than 8 hours a day, you are to be treated as any other employee and not singled out for special treatment. If you don't want to work overtime and/or if you feel you are unable to work overtime, then I suggest you have your doctor write a note to that effect and you hand it in.
A lot of working women "fought" long and hard to be able to keep working while pregnant, not because we really wanted to, but because our money bought the food and paid the utility bills at the time. It wasn't all that long ago when pregnant women were forced to quit work or had to use the "back hallways" so as not to make visitors and customers ill because they had to look at a pregnant woman walking down the hallway. Times have changed and we certainly don't want them to go back to the way they were now do we?