Any non-pressurized aircraft will stay at low altitudes. The air pressure in the plane will be no lower than you'd experience walking around Vail, Colorado, or any number of other mountain towns - not all that much different from Denver. I don't recall anybody advising Denver women, or even Vail women, to move to the lowlands until they deliver.

I'd at least check with another doctor.

(BTW, an aircraft's service ceiling is a technical function of its engines, wing area and things like that. Whether or not it's pressurized is based on a whole different set of considerations. Nothing says it has to fly anywhere near its service ceiling, and there are such things as oxygen supplies for the crew if they have to fly a non-pressurized aircraft over mountains.)