Boss won't allow time off

Is it legal for my boss to refuse time off for prenatal dr apts?
This is so sad to me that I work for a medical institution and my boss won't let me go to my dr apts. unfortunately I work 10 hour days and every obgyn in the area sees their last pt at 4:00pm I'm just trying to leave a little early or come in late and they are not being cooperative at all... Anyone know anything about this I live in California.