Fanfiction.net is also a place to check. There are many different types of fanfics there, and with a proper search using the keywords 'usagi pregnant', you might find some interesting stories. However, the quality can vary, so you may need to sift through a bit.
Some 'usagi pregnant fanfic' could explore the relationship between Usagi and her partner during her pregnancy. It might show how they grow closer as they anticipate the arrival of their baby. Maybe they have cute moments like shopping for baby clothes together or discussing baby names.
One possible plot could be the two characters preparing for the arrival of their baby. They might face challenges like finding a suitable place to live as a family, dealing with family reactions, and preparing nursery items.
One great 'Usagi Drop' pregnancy - related fanfic might be the one that focuses on how the characters adjust to the new addition in their family. It could explore the emotional journey of the main characters as they prepare for the baby.
I'm not sure which ones are the most popular specifically, but you can start by looking on fanfic websites. Check the tags related to 'Usagi Drop' and 'pregnancy'. There might be some fanfics with high ratings or a large number of reviews that could be great.
Love and sacrifice is a common theme. Mamoru would sacrifice his time and energy to take care of Usagi. Another theme could be the power of friendship. Their friends would likely come to their aid during this difficult time. Also, the theme of hope. Despite Usagi being sick, there is always hope for her to recover and have a healthy baby.
The plot could involve Usagi's sickness being related to some sort of magic or enemy plot. Since she's pregnant, her powers might be affected. Mamoru, being the loving partner, searches for a cure while also protecting Usagi. There could be a lot of emotional turmoil as Usagi is worried about the baby and Mamoru is torn between finding a solution and being by her side all the time. Eventually, with the help of their friends, they figure out what's wrong and Usagi recovers, ready to give birth to a healthy baby.
Another place could be FanFiction.net. However, the availability of such a specific fanfic depends on whether someone has written and uploaded it there. You need to create an account if you want to access all the features and search thoroughly.
One great 'usagi and misaki pregnant' fanfiction could be one where they navigate the challenges of pregnancy together in a modern AU setting. They might face things like telling their families and friends, and preparing for the baby's arrival. The story could focus on their growing relationship as they become parents - to - be.