I have a baby with an abnormally large head, cab I have natural birth?

Maybe. The head is the largest diameter that must pass through the birth canal at delivery. If pre-delivery estimates doubt the head can pass, a scheduled c-section is common.A trial for a head down delivery might be done, with a set up to do a c-section immediately if the baby showed stress. Since a big head could get stuck, a breech delivery would be so dangerous most would never consider it.