I'm not sure if there is specifically such a fanfiction. Fanfiction can cover a wide range of themes and ideas within the Pokemon universe, but pregnancy themes might not be as common. You could try searching on some well - known fanfiction websites like FanFiction.net or Archive of Our Own to see if any exist.
Honestly, I don't know if there are Pokemon Paul and Dawn pregnant fanfictions. Fanfiction writers often come up with all sorts of creative ideas. Some might focus on their relationship in a more romantic way without involving pregnancy. Others might explore different aspects of their characters like their growth as trainers. It would be best to search through fanfiction communities or use search engines dedicated to fanfiction to find out for sure.