Teen mums and social work referrals

[reprise]

I didn't want to derail the other thread about the teen mum who seems detached from her newborn, so I'm starting a new one here.

On another messageboard on which I post a user has expressed some concern about the social circumstances of her 18 year old sister who is pregnant and due to deliver in a couple of months. When I mentioned that social work referrals are SOP for all teen mums here, I was advised that once someone is over 18 in the US this is no longer the case. Now reading the other thread makes me think that in at least some US hospitals such referrals are still made on a routine basis (although I assume that an adult is within their rights to refuse the consultation).

This particular teen is caught in the middle between two families with totally different ideas (the father of the baby is a JW) and the potential for things to get extremely ugly if anything goes wrong during L&D is extremely high. The young woman's family are working on the assumption that they will automatically be regarded by the hospital as NOK (the two young parents are not married), but nobody seems to have clarified that or enquired about who gets to make decisions if the young woman herself is unable to do so.

So I guess my question is twofold. Under what circumstances are those young women who are over 18 referred for a social work consultation, and are the young woman's family correct in assuming that they will be the ones legally entitled to make decisions on her behalf if she is unable to do so at any time during labour and delivery (and if not, what steps need to be taken if the young woman concerned wishes to formalise this)?