Former social worker speaking. If no one makes a referral (i.e midwife, relatives, teachers if you are in education) then they won't get involved. The midwife is not going to automatically refer you because of your relatively young age, it would be something else like pre-natal depression and a lack of support combined with your age etc. Don't mention being Autistic to the midwife if you don't feel that you need to. If you feel it won't effect your pregnancy or ability to cope then you don't need to mention it. Lots of women are never diagnosed, they might be a bit quirky or seem introverted but tbh I don't think it effects their mothering anymore than any other range of personality dimensions. If your children are neurotypical you don't really need to teach them social skills to the same degree you would an asd child anyway, they just get it, just teach them good manners.
If you are not / have not been in care and are not already on the system then it is unlikely you would flag up as in need of intervention. If they became involved due to some other concerns then your diagnosis may become an issue at that point as part of a bigger picture but on its own not so much. It would also be fair to point out that to judge and discriminate against your mild differences is deeply unfair. Your are neuro atypical not disabled, you think differently, that's all.
If they do become involved just emphasis how much support you have from wider family and your boyfriend and his family and seek out support and positive influences to help you through. Also emphasis your coping strategies, emphasis that autism can make your MORE focused on baby's needs because of a natural desire to create routines and learning about topics of interest in depth etc. Show readiness and excitement for the baby both mentally and in terms of having the right equipment to hand early.
Have sensible life goals for you and baby (and your boyfriend too) that you can action easily and realistically. I.e how you can incorporate work/study and childcare.
Having social services involvement doesn't automatically lead to a care plan, they will often just want to assess your circumstances and reassure themselves you are young but determined and caring. If they do become involved go along to everything, prenatal classes, baby groups etc. If they aren't then just do what feels right. Some professionals are quick to judge and you can get a feel quickly for who they are, it might be the prenatal yoga instructor etc lol. Tbh I would avoid judgey people at this time.
Also, slightly off topic, as for going to groups etc, other people at groups etc don't need to know how old you are. Some 17 year olds look like they are in their 20s and some 20 year olds look like they are in their teens. Nobodies business really.
I will say that unfortunately as a teen mum you will be held to a higher standard! Just make an effort to do the best you can do and take help that is offered or demonstrate why you are just so awesome at parenting that you don't need it.