It depends on how willing the other party is to accept that name as your legal identity. Generally speaking, under the law, you can use whatever name you want, so long as you aren’t trying to defraud anyone.
You can use as many different names as you want. If your name is “Christopher,” then you might have intimates who address you as “Chris,” “Christy,” “Topher,” and “Kit,” among other things. It’s all legal.
Most American women change their names legally at marriage. (In some countries, your legal name never changes.) There’s a space on the form to allow either or both parties to a marriage to do that. But you don’t have to. A lot of women change their legal names but keep their maiden names for professional use. Everything’s allowed.
The U.S. system prohibits fraud. People use different names all the time throughout their lives.
To be fair, some of those “disappeared” children might have existed, but their parents had never bothered to get them SSNs. Procrastinating like usual, it was then too late come April 15th to do so. I wonder how many of those 7 million, reappeared in the next few years as this was corrected.
The default name here is “Infant”- I’ve treated a few babies who were still going by it at 5 and a bit weeks, but all had proper names by the 6 week deadline for birth registration. I generally addressed them as “Sweetheart” or “Buster” because I don’t like having nameless patients.
IIRC not registering a birth (and getting a birth certificate) in the UK would not only cost you a fine, but would leave you open to the charge of concealing a birth, which is a crime.
I have three kids who were born in the mid 1990s in New York.
We did not name our children before they came home from the hospital. Our sons were not named until after the bris and my daughter was not named for five days. Their birth certificates have blank spaces on the front for their first names but are stamped on the back with a stamp stating that their name “X” has been added to the official copy on file.
Of course that was quite a while ago and pre-9/11 (I don’t know if that changed anything or not).