If a residency can't accommodate a resident who is pregnant/hurt/has a family emergency/etc., then that is a serious problem for the residency. It isn't hard to reschedule people and it isn't hard to make up shifts if needed. If it is too much of a problem for your residency, then you probably need to look at restructuring your program and/or whether you actually have the capacity to have one at all. Worst case, a department with attendings should be able to pick up a bit of extra work if a resident, their replacement resident, or a PA/NP can't come in.
I know of a resident at IU who was recently able to not only convince the EM program that residents in their first and third trimesters shouldn't do nights (unless they wanted to), but every GME program in the hospital. You never know when you will need help, supporting each other is part of that. Scheduling life doesn't get that much easier after residency, so don't put your life on hold in hopes of greener pastures.