When it comes to sex-while-pregnant, Sisterly Lust gets my top spot. However, it is a bit weak in the leadup to it. There is a barely disguised "knock this one up yes/no?"-prompt, a timeskip, and then they are pregnant. I would have loved some more context to, some development leading to a conscious choice for the girls to become pregnan, stuff like that.
Parental Love did some of that nicely, although I would still want more.
A more grounded but still great one is Reunion. Just one pregnant LI, but there is all that context.
Peasant Quest definitely deserves a mention, too, for sheer scope and variety.