I don't think you are ever "allowed" to speed as such, there is no written law that you can break the speed limit if there is a pregnant woman in your car, however you will often find that some offers will have a certain amount of discretion when they pull you over. I know if we see anybody who needs to get to hospital, we will give them a police escort so long as we are able to do so, as it is much safer with the sirens - or we will try to contact an ambulance. Our main priority is to make sure that everybody is safe on the roads, but if somebody needs to get to hospital then that's important, too.