Working hours

So I work 40 hours a week (over 4 days so 10 hours a day with half an hour break) as well as on my day off attending college for 6 hours and was just wondering what the maximum would be when pregnant being on my feet all day working in a very busy nursery, I understand there are risks and I'm only 7 weeks pregnant and yet to tell my employer but wanted to educate myself on my rights... Any help would be great!