While True Polymorphed, say into a dragon, can you get pregnant? If so, what happens when you revert form? I am a player in a D&D game that wants to know.
A precedent exists for the answer to possibly be 'yes', but this is still very much a case-by-case/DM's choice basis.
The entry on half-dragons in the Monster Manual cites that they come from (in some cases) a coupling between a dragon polymorphed into another form and another creature (presumably one compatible with the polymorphed form). This would imply that, for dragons at least, pregnancy is a possibility and results in a hybrid between their true form and the form of the partner.
It does not delve into the topic of what that pregnancy is like as it shifts between forms, however, so that's up to your DM if you wish to broach that topic with them.
Only your DM knows
The rules of D&D 5e do not cover pregnancy. It is a story element, not a game mechanic. As such, it is entirely decided by each DM.
Do dragons even "get pregnant" in your DM's world? The Monster Manual mentions eggs, but are they laid fertilized, or is that done afterwards? Do dragons even have 2 sexes? We can safely say that they don't follow our world's rules of physics, why would they follow its biological trends?
Without input from your DM, we can't even have a useful discussion about how pregnancy interacts with various spells. This isn't even just the case with dragons. How does interracial mating between different humanoids work in your DM's world? We don't know.
You'll have to ask your DM.
If there is a mention of the occurrence in any literature that is core, then it sets the stage to argue for the possibility with your DM. You should be able to keep it in the story and have your DM roll up specs for duration, gestation, and complications via chance modifiers, based on assumptions and preferences, unless you are just leaving it a backstory element to explain a hybrid PC who is easily more functionally creatable by listing balanced abilities that he/she would agree to allow them to have. Wouldn't bring it to a convention without a ton of research and the precedence citation (such as the entry on Half-Dragons) but in personal home games it should be totally fine.