Thinking about big boobs with dark nipples ...
and as far as I know, the pussy also changes a bit during pregnancy.
Also, I heard that due to the change in hormones, many pregnant women can become very horny.
As for a story idea, I have 2 ...
1: what about this: there is this woman, who is like 8th or 9th month pregnant. Maybe it was a one night stand, or maybe the guy who made her pregnant ran away, when she told him about her pregnancy.
She is alone, so she has to think about ways to make money. One friend of her suggests modeling for an art class. Being pregnant shouldn't be a problem, since it is about art, not porn.
In the past, she would have never posed nude in front of others. But now, in her pregnancy, the thought of being totally nude in front of many strangers make her horny.
She even shaved her pussy, even though that was very hard with the pregnant belly (either she used a mirror, or she asked her best friend to help her).
She enters the art class, dressed only in a robe. When she opens her robe, to reveal her body, some guys (and girls ?) can't stop to stare at her boobs and pussy. But then, they start to draw pictures. The woman loves the attention.
During a break, some guys start to talk to her. The woman is pretty open and tells the guys that she is single. She even tells them, that the last time that she had sex led to her pregnancy - so she had no sex since about 8 months.
2: This pregnant woman is posing as a nude model in the art class. However, there is also a male model. He is pretty young and insecure, but he also enjoys the attention, especially the ones from the female students (or maybe it is an females-only class).
When he sees the pregnant model, he can't help himself, he has to stare at her big boobs and her shaven pussy. She notices that and smiles at him - and suddenly, the guy gets an erection. He is embarrassed about his erection, but the female teacher says he shouldn't be embarrassed. An erection is just a normal thing - and that way, the class can also draw pictures of an erection..
After some time, his erection becomes limb, but the pregnant woman teases him just enough to make his dick hard again. At first she just plays with her pussy in front of him, but after some time, this ain't enough, so she starts to suck him, but only a little bit. It's pretty strange to do a blowjob in front of the art class, but the guy enjoys it - and so does the art class ...