I fly a turbo 182 which is not pressurized and the service ceiling is 20,000'. The above post about service ceiling is correct.

As are the comments that in pressurized aircraft the cabin altitude (the pressurization system does not pressurize to sea level pressure)--generally, cabin altitudes are at or below 8,000'. Think Colorado ski resort altitudes.

My wife is currently pregnant and we asked our doctor about flying. The issue of flying unpressurized is not about oxygen content provided the pilot is complying with FAR (Federal Aviation Regulations). The issue is about rates of change.

The standard climb and descent rate for cabin pressurizations is 500'/min. The real rates of climb and descent in pressurized aircraft are much, much higher (as much as 1000-3000 ft/min, depending on the aircraft). This lower rate is tolerable by most all individuals (except allergy suffers and crying babies, hence the crying). Cabin altitude changes (or real altitude changes in a non-pressurized aircraft) exceeding those rates could distress or damage the integrity of the amniotic sack (the bag of tissue containing the fluid your baby is swimming in).

Therefore, our doctor limits our flights to a 500 ft/min max rate (we currentlyt have a low-risk pregnancy). Most pilots flying a non-pilot passenger would limit their climbs and descents to this (500 ft/min) anyway for general comfort. Something about unexpected wild rides don't encourage repeat passengers.

Obviously, if you have experienced altitude sickness before, you may want to reconsider your travel plans.

Additionally, I am not a doctor, and my doctor is not a pilot, but you may want to ask him/her about climb rates, etc, not just altitude.