Should I tell my boss?

I literally just got a new job, had my first shift on Monday. I’m 4 weeks and 5 days today, I’m wondering when do you think an appropriate time to tell my boss is? Should I wait until my first scan? I’m going to call the doctor today so I can book the time off work. (I’ve never booked time off so soon 😬) Not to mention its an hour 36 minute walk to work.. I’m doing fine now but that’s a lot on a pregnant person there and back!!! I would like to ask for shorter shifts if possible, I’m just nervous how to approach the situation 🤔😅 advice??