A NEIGHBOR in her 30s, a very fine woman, recently had a child with her boyfriend. They live together.

The boyfriend’s mother and father enjoy being grandparents. The boyfriend gets to have a son, as well as live-in female companionship. How eager he was to be a father, I don’t know, but he clearly doesn’t want to be the mother’s husband. He’s keeping his options open.

Meanwhile: She works full time; baby care is totally her problem, and her day is a streak of maniacal multi-tasking, from the 5 a.m. wakeup until she drops into bed at 10 p.m.

Nearly 40 percent of babies now born in America are to unmarried women. The birth rate for unmarried women is higher for women in their 20s than for teens.

Sociologists say that these new mothers often assume that the baby will “cement” the relationship. But these arrangements rarely last. Some single fathers no doubt gallantly pitch in. (And some married ones don’t.) But that’s not the norm. Most such single mothers become beasts of burden.

A PBS NewsHour segment about “people hard hit by hard times” featured Heather Scharf. A 37-year-old single mother, Scharf recently lost her well-paying job. She’s scared, stressed and depressed.

We learn a lot about her: She dropped out of a community college at age 20, then worked her way up in the mortgage business, and spends $800 a month on a school where her daughter can stay during her late hours at the office.

But there’s not a single word about the father. Was he married to Heather? Is he paying any child support? Where the heck is he?

The story of the struggling single mother has become so commonplace that few even question why she’s doing this so utterly alone. There should be a standing newsroom rule that all such accounts include some reference to the fathers.

I wonder most about women like my friend. She was not an impoverished teen lacking a traditional family model. She was not one of those well-to-do professionals buying the sperm of an anonymous high-achiever. She belongs to the hardworking middle class. In olden days, she would have had a frilly wedding before starting a family. (She did have a baby shower.)

Perhaps I’m making too much of marital status. After all, many divorced mothers engage in similar toil. For the legalities, I spoke with Barbara Glesner-Fines, a professor of family law at the University of Missouri- Kansas City.

Regarding children, the law treats the father (or mother) who breaks up with the other parent much the same whether there was a marriage or not. A court will impose the requirements of support and privileges of custody and visitation.

The difference is that when a child is born during a marriage, the husband is presumed to be a father. That is not the case when the couple is not married. “He has neither the rights nor responsibility until he is found to be the father,” Glesner-Fines said.

I ask a sociological question: Does a marriage intensify one’s sense of duty?

“Formality in the law serves some important purposes,” Glesner-Fines responded. “It cautions people that what they are getting into is serious.”

Yes, that’s it. The seriousness of the legal bond between the parents — as well as from parent to child — helps foster a partnership in child-rearing, even if that bond later dissolves in divorce. Why so many women take on motherhood without such formality in place is a mystery. The sad result is a growing sisterhood of drudgery.

Froma Harrop is a member of The Journal’s editorial board and a syndicated columnist.