I've enjoyed these in personal relationships in the past; but curious if there's a provider who would as well. For obvious safety/health/risk reasons, this boils down to only consisting of some quite explicit and specific dirty talk. The role play can be expanded upon with 'forcing' or degrees of submission for the latter taking it to different levels.
The dirty talk would first be centered around how there isn't a condom being used (but, as mentioned, for obvious reasons there would be) and she isn't on birth control. It might conveniently be during her ovulation cycle (guesstimated from her most recent actual cycle). Regardless, she just can't stand to waste any, have you pull out, willing to risk it for the feeling, etc.
For the latter fetish, much of the above is applied; but more certainty and talk is directed towards how she wants to be with child and craves you to make that happen. Talk about how things would be while she was pregnant and how she wants to be kept that way. This could be taken (or said) that to bare children is her purpose in life and that she is meant to be 'used' to produce offspring.
To do this with an already pregnant provider has potential to take it over the top, and introduce new areas of talk with more visual stimulus from her already being with child.