They can't and don't.
How girls get pregnant by girls?
They can't. It is biologically impossible to get pregnant without one or the other.... Sperm and egg! If you see two females with a child then the child was either adopted, kidnapped or one of the females found sperm from somewhere!
If that were possible, they'd have killed off all the guys by now.
(Except, who'd kill the spiders and open those pesky jars?)
It happens when the fertility doctor that preform the artificial insemination is female. Though I wouldn't call someone old enough to have gone through med school and establish a practice a "girl". I think woman would be a more appropriate term.
that's impossible.
Artificial insemination.