so recently Eberron: rising from the last war came out, and with it an official version of the changeling player race, the only race in dungeons and dragons that can change their sex. Now, considering that changelings have existed in the world of Eberron since like at least before the last war, and considering the fact that changelings age at the same rate as humans, it is implied that changelings can reproduce. Now since changelings are genderless in their "natural" from they are presumably forced to use one of their masks or one of their personas in order to reproduce, and here is my question: what happens if an female mask or persona belonging to an changeling gets pregnant, and the changeling changes to some other form before changing back? 

also, what happens to stab wounds when an changeling shifts to hide the wound? does the stab wound still exist? are they still hurt? How exactly does the shifting of changelings work? Is it changing flesh? what happens to internal organs? How sofisticated is the changes made? Will any of this come up in an swashbuckling noir intrigue plot?

Most games don't have reproduction as a thing that the PCs need to worry about, so it isn't detailed. For your game, if it matters, work with the player or DM to come to an acceptable solution. Otherwise, Because Magic should suffice.

As for the rest, the key takeaway is your game statistics do not change: Your hit points, AC, attacks, spells, everything other than appearance, are unaffected by shapechangeing.

what happens to stab wounds when an changeling shifts to hide the wound?
It probably still exists, decide with the DM / player if the shapechange feature can cover it up

does the stab wound still exist?
yes

are they still hurt?
yes

How exactly does the shifting of changelings work?
Like magic but its not. Its just like the doppelganger, but medium sized only

Is it changing flesh?
Think of it like the outside flesh only

what happens to internal organs?
They probably stay the same

How sophisticated is the changes made?
It says so in the Shapechanger description: genderswap, hair style change, skin color change, minor height / weight change (still Medium sized). Your game statistics do not change, so you might be able to cosmetically have gills, but they would not let you breathe underwater. Work with your DM / player to determine the limits.

Will any of this come up in an swashbuckling noir intrigue plot?
Sure, if you have to infiltrate anywhere, this can be very handy.
Edit: being able to potentially hide wounds and shapechange

As far as wounds go, I'm assuming WotC didn't release an official race that has unkillable insta-heal powers. In practice that's way too unbalanced and cannot work. In fiction I would say that the mask they wear when they shape change is only skin deep. If a changeling gets stabbed and then transforms, then they're now a perfect duplicate of someone except they still have a stab wound. 

As far as reproduction goes, I don't recall anywhere it stating that changelings in their natural forms are genderless. I had always been more under the impression that they only *appeared* to change their sex when they transformed, but internally remain the same (maybe with a few anatomical adjustments depending on how NSFW your game is).

Also, changelings are not the only race that can change sex. According to Mordenkainen's Tome of Foes, certain elves are able to change sex due to a blessing from their god, and in that case it us described as a complete bodily transfer, and not just taking on the appearance of it.

Looking back at the book, it says that "Changelings have a fluid relationship with gender, seeing it as one characteristic to change among many."

The book does not mention sex, so I would interpret it to say changelings do not confirm to binary gender expression typically, but do have a physical sex for the purpose of reproduction that likely does not change despite gender fluctuations. 

i could've sworn that the ua mentioned something about being genderless in their true form and how that shaped their world view, but now that i went to see it again the information does not appear in the article, so i donno maybe 

And who is to say Changelings even birth young instead of something more novel like acquiring fresh genetic material, metastasizing a tumor or egg-like structure, and then just budding or laying that flesh-orb in a slime bath to hatch and develop on its own?

According to Keith Baker, the idea has historically been that changelings lose the ability to shapeshift while pregnant. However, he’s suggested that it’s certainly possible for a changeling to learn enough control over their abilities to change their outward appearance while keeping their uterus etc. to avoid complications.

Regarding wounds, it depends on the severity of the wound. A changeling can’t hide a gaping hole in their chest, just like how they can’t regrow severed limbs, but minor cuts, sure. It just doesn’t heal any damage.

Quote from SagaTympana >>

Regarding wounds, it depends on the severity of the wound. A changeling can’t hide a gaping hole in their chest, just like how they can’t regrow severed limbs, but minor cuts, sure. It just doesn’t heal any damage.

And you usually don't get a serious wound until you hit 0 HP anyways, at which point you're very likely to either die or receive magic healing.

Yeah, this isn't an area of lore which the devs go into detail about. It would probably make more sense to get inspiration for sci-fi movies and books, honestly.  If you want to make it so that Changelings have two sexes, five sexes, or no sexes, that's all up to you.  Look to stuff like Star Trek, for instance, or research reproductive biology in brahminy blind snakes or aspidocelis lizards.

Also, please note that gender is not the same as sex.  Just throwing that out there to minimize the use of incongruent language and any newbie mods who think I am somehow using NSFW language here.  If we talking about biological reproduction as being related to bodies, then we are discussing sex.  If we are discussing behaviors and socially-productive/generative roles related to, but not always directly correlated to genetic continuance of particular traits, then we are talking about gender.  

houligan i cannot think of an intrugue plot where it would be benefitial to be pregnant for 18 months, especially when you are a shapeshifter and can just pretend to be pregnant

ah yes, was gonna say something about changelings maybe cloning but did not feel it fit the post, especially since doppelgangers do it the "normal" way

SagaTympana can you send me some kind of link or source for that?

also thanks

I mainly meant as a changeling in general.

Any of the reproductive stuff probably doesn't matter, and the wounds stuff may help, if you are for example trying to run away from guards.

You could have them reproduce like the changelings in Ster Trek:Deep Space 9.

I once read an issue of I forget what where Mystique (the marvel character) used her shape shifting ability to move her organs into her legs to save her life from a hail of bullets. 

Quote from ArtificeMeal >>

SagaTympana can you send me some kind of link or source for that?

also thanks

Here you go.

I think the wound stuff will all be DM discretion, since there are also no rules per RAW for actual, physical wounds. There are *optional* rules in the DMG for injuries, but they're non-standard.