River has two mommies



Since we know that the TARDIS is female ('The Doctor's Wife'), and River Song is the child of the TARDIS, that means that the female TARDIS, er, 'joined in' on Pond's wedding night, when River was conceived. That makes both Amy and the TARDIS her mothers.
Regardless of whatever other criticisms we might have of Steven Moffat's writing as show-runner, I think that most of us can agree that Moffat deserves high praise for this one. :-)

.