Yoo. I'm looking for a twidash story. I don't remember the title. But I do remember the story is about twilight getting pregnant through a spell she put on rainbow during a drunk night and it involves a stigma about mares getting another mare pregnant through that spell.

Do you mean this story?
https://www.fimfiction.net/story/104086/rainbows-in-the-night-sky

Yeeeaahh that's the one!!!!