Best friend's serrogant

My best friend (since 1st grade) is a year older than I. But is what we call a late bloomer. I'm pregnant with my third child, engaged, and living on my own, with a career in place. And she's just got her second bf. She's always dropping hints on her jealousy of me being a mom. Grant it I had my first child at 17, nothing to be jealous about. But according to her medical history, she was born a conjoined twin, she cannot have children. In high school i told her I would be her serrogant mother when we're older and now since I have going on three kids, I honestly don't think I could carry another child, let alone give it away. Idk how to tell her. We're so close I know it hurts everytime I pop up pregnant 😔 thoughts ladies?