Echoing what most have said here-your boundaries are quite reasonable. You may have a handful of potential clients chose to go elsewhere, but it sounds like you're busy enough that you're not begging for business.
I had a situation just a few weeks ago where a client who is in a family business can ONLY have evening appointments. (Apparently this business will fall apart without her.) My office hours are rountinely until 7 pm, last appointment at 6:30. She had made a 6 pm appointment, I think, because it was the only opening available. So she just showed up at 6:30, when the woman who had a 6:30 appointment was already there. I couldn't reschedule her ANYWHERE else, so I stayed late and ended up being late for something in MY personal life that's important to me.
Through an interesting set of circumstances, she was responsible for keeping my partner over late on another appointment last month.
I've prepared a speech of sorts when she comes in for her next visit: if she wants the benefits of a small and intimate practice, then she needs to be willing to accept our limitations also-and office hours until 7pm two days a week is not a severe limitation. If her schedule is so inflexible that our hours don't work with hers, then she needs to find a bigger practice with more extensive hours.
I have no issue going above and beyond for women who have a problem or complication. But persistently keeping us over our scheduled hours for a normal pregnancy is not a trend I'm willing to continue.
Thanks for letting me vent, gang.

nashvillemidwife, you are a commited professional who is providing a highly specialized service. Congratulations on recognizing what you need to do to stay happy and healthy, and for averting burnout before it happens.