My Grandmother is the youngest of six children and the only one still living.
Like myself, she was highly amused, when I discovered that she was the only one born in wedlock!

On all six of the children's birth certs, my Great-Grandmother is recorded as 'formerly....' and my Great-Grandfather appears on all of them too.
My Great-Grandmother wore an engagement and wedding ring, and none of the living members of the family knew about the marriage - which took place 11 years after the first child was born, and not in the local church, but the registry office, of a neighbouring town!
It's my impression that it was quite easy to pull off the pretence of being married. I don't think the Registrar would ask for any kind of proof??
Kind regards, Glen.