They could also trip while going upstairs to bed. Or down the stairs to on their way out of work. Or in the grocery store. Using your logic, pregnant women should basically go on bed rest to prevent all of the potential accidents that might befall them.
It is actually worse for the mom and baby for the mom to change her exercise routine after becoming pregnant. Obviously, pregnant women need to be cautious, but with a doctor’s clearance, running can be great exercise when pregnant.