Your experience with your trans friends is wholy different than mine. Maybe its regional or cultural but the FTM trans I know well wouldn't object to being referred to or lumped in with pregnant women on a general basis and if they did they are fully capable of correcting those refering to them with whatever term suits them. For non-binary pregnant person may sufice, for some FTM they may prefer pregnant man.
Thats kinda my main problem i think. Health care is such an individual thing. There's absolutely no reason why a doctor, nurse, or receptionist cannot use whatever term the person in front of them prefers instead of the extremely illiteritive term pregnant people. Why not just say the Pregnant? Or heck pregnant individuals if addressing a group? Pregnant people just sounds strange on the tounge and doesn't really suit everyone. As a cis female person I want to be referred to as a pregnant woman not a pregnant person. Your friend maybe would have preferred pregnant man? Idk but why can't we just meet individuals as individuals that they are?