I remember reading a story a few years ago, so a lot of the details are a bit fuzzy...

It started out with a slug (I think it was alien?) crawling up into a girl's womb and starting to reproduce. A while later the girl realized she was pregnant with these slugs until she grew to be 9 months pregnant sized and gave birth to them.

Does that ring a bell for anyone? Any help is definitely appreciated!!

Komperaclause on deviantart.com. "slugs". There's a male and female version.