Scheduling Dr. Appts

How do you schedule pregnancy appts while working a mon-fri job? I have had to re-schedule multiple appts so far because my manager will say it doesn't work for me to leave early or in come in late. She tells me I need to switch shifts with someone and work a late evening shift or switch someone and work the weekend shift on my appt weeks. It has messed up my schedule and I am exhausted. If I work late night shifts it messes up my sleep and if I work the weekend then my mon-fri after it puts me at 6-7 days in a row. Im not sure how to manage this. I am told that it is an unplanned absence if I do not put in a PTO request. All PTO requests have to be in 30 days in advance. Do I just schedule all my appts months ahead of time? Some are hard like I need a 2nd ultrasound about some concerns and they want to do it asap but I have to reschedule because my manager said the next week or two works better because we will be less busy.