^^ Not sure where you're getting this information from mate, because pregnant women CAN work, and they frequently do. A lot of pregnant women work all throughout the pregnancy until about 1 month to go. And some crazy loons work all the way up until just a few days before the due date!
But especially prior to about 6 months in to the pregnancy, sure some (most?) women will be much more tired than usual and some are often really sick with morning sickness, but most of the time this doesn't actually require them to not work at all. Obviously if the woman's job is particularly labour-intensive or subjects her to dangerous situations etc, then they will most likely not be able to continue doing that type of work. But in most situations working isn't an issue for pregnant women.
Obviously as the pregnancy gets closer to term there are many reasons why a woman might reduce her work hours, or stop working altogether. The extra weight of the baby puts a lot of strain on the body so they tire easily and need frequent naps throughout the day. There are also issues with fluid retention in the legs so they need to elevate their legs/feet frequently to try and alleviate the swelling. There are other reasons too.