fetus friends 

2 or more friends who are unconditional best friends from birth, who know each other inside and out. they're usually kind of annoying

those two are fetus friends

fetus friends by Adam Henaghan August 23, 2007

Fetus friend 

A friend that you’ve known since the infancy or a childhood friend that you’ve known your entire life, essentially since the time you we’re both fetuses, because your mom and their mom and/or parents have been friends since before your birth and you became friends with your parent’s friend’s child by default.

I was hanging with my fetus friend last week, the one I knew since I was basically a fetus.

Fetus friend by Fetusfriends101 June 13, 2019