Bless your soul for taking in this kitty and giving her a loving place to have her babies and then a permanent home.
She looks lovely, aren't red females more uncommon than males? She does look huge, but sorry I also have no experience with pregnant kitties. I agree that a vet visit may be in order if you really don't know when she's due, you wouldn't want to take any chances on her having trouble and not knowing it. Hugs to you for being such a caring human!