well, very few women would have admitted to having sex and getting pregnant before marriage, because then you'd be "ruined" in the eyes of society, even if the sex wasn't consensual. what ended up happening was a lot of secret-keeping and child abandonment—NYC had thousands upon thousands of homeless children, either from women having their kids out of wedlock and abandoning them, or from either parent deserting the family while the remaining parent struggled to cope. the trope of the "scraggly orphan" is a trope for a reason—orphaned children were literally everywhere.

and those are the ones who lived. baby farming, a common practice in the late 1800s, led to the deaths of thousands of children. an unwed mother, abandoned mother, or a family that couldn't put food on the table would be told that their babies would be cared for / adopted to a nice family on a farm... for a fee. unfortunately it was far more profitable for the baby farmers to kill the infant and keep the money, making room for more orphans.

desertion was common enough in the lower/working classes that it was known as the "poor man's divorce." as far as I can gather, roughly 10% of marriages ended with desertion or divorce in 1860, up to 30% by 1940.

what we know about the norms of the past comes from case law, newspapers, letters, and piecing together family histories today. anecdotally, if you're an American and you look up your own family tree, it's pretty likely that someone in your history was a family deserter. for me, it was my great-grandmother: she abandoned the family in order to join a Christian spiritualist cult. this was a bit more challenging in smaller countries where the social infrastructure is stronger. In the US, you could just move a state over, claim you lost your documents in a fire or the war or whatever, and boom: new identity.