Hey, what can I say. I was really, really drunk that night.
No she was really, really, really drunk that night.
I'm firmly convinced that the father is her son.
Call me twisted if you like; my SO did when I proposed this theory. I'm still of the opinion that it's the most satisfyingly repugnant resolution.
Now that would be something. It would explain why the son OD'd when the kid was born and why they never show any pictures of it. Maybe the baby has twelve fingers or two heads.