Pregnant women at work in the United States can be seen as a liability to some working environments and while they won't be fired directly for their medical condition it is possible that employers could fire them for other reasons during their pregnancy. On the other hand, it's far harder for an unemployed pregnant woman to find a new job because her position is obvious and employers do not want to give time off shortly after hiring someone.
I think sometimes this is a two way street. I have heard of many women being pregnant during interviews and refusing to warn the employer that they will be taking a significant leave as soon as they start. I think this is as bad as firing a woman because shes pregnant. Both sides need to be honest will each other
It is very rare indeed that a woman gets fired simply for being pregnant. The majority of these cases are in looks based jobs like modeling, dancing, etc. A few backward employers may be still trying to fire women for being pregnant. However these types of cases are increasingly few and far between.
Contrary to what many people may think about pregnant women about work and pregnancy; they are very good at what they do and take pride in working hard It is true that they may get tired but that is fine and if you was pregnant you would be too. One thing you can be sure is that a pregnant women is just as good a worker as anyone else.