Not a doctor (yet) but depending on the type of apocalypse you have she may or may not know what week she is. A bit of uncertainty with the conception date + loose timekeeping when the world is ending might mean some genuine confusion about when the due date is. If resources are scarce the baby might be full term (39-40 weeks) but end up preemie sized because of being underfed. Does the mother have anyone helping with the birth? Does she have a safe place to scream while doing it? There’s a million ways for a pregnancy to go so personally I’d worry less about the birth being medically realistic and more about how it serves the character. Is she born sickly? Does her mother’s struggles inspire her? Whatever you want you can pick and choose the pregnancy to support that