Despite the anxiety society still feels about single mothers, most American mothers aged 26 to 31 had at least one child when they were not married. And the number of these millennial single mothers is increasing. In fact, in a study just released by researchers at Johns Hopkins University, only about a third of all mothers in their late twenties were married when all their kids were born. And two thirds of them were single when at least one of their kids were born.

The less education the young women have the higher the probability that they became a mom before they got married. Conversely, the married moms of that generation probably have a college degree. “It is now unusual for non-college graduates who have children in their teens and 20s to have all of them within marriage,” says Andrew Cherlin, one of the authors of the study “Changing Fertility Regimes and the Transition to Adulthood: Evidence from a Recent Cohort.”

Sociologists such as Cherlin have been tracking the decline of marriage as one of the milestones or goals of an individual’s life—the whole “first comes love, the comes marriage, then comes the baby with the baby carriage” paradigm. And it’s clear that an increasing number of young people are just not putting a ring on it. “The lofty place that marriage once held among the markers of adulthood is in serious question,” says Cherlin.

Motherhood is beginning to show the fissures along income and education lines that have already appeared in other aspects of U.S. society, with a small cluster of wealthy well educated people at one end (married with kids), a large cluster of struggling people at the other (kids, not married) and a thinning middle. While many children raised by single parents are fine, the advantages of a two parent family have been quite exhaustively documented. Some of these advantages can be tied to financial resources, but not all.

Among people with kids between the ages of 26 to 31 who didn’t graduate from college, 74% of the moms and 70% of the dads had at least one of those kids while single, Cherlin found. A full 81% of the births reported by women and 87 % of the births reported by guys were from people who didn’t finish college, so some of these single, lower education parents had more than one kid.

The chart below, using data from the National Longitude Study of Youth 1997, which looks at people born 1981 to 1984, shows all the births reported by women who didn’t get through high school, how old they were when their kids were born and whether they were married. Only a quarter of these young moms were married, slightly more than a third were living with someone, not necessarily the child’s father, and almost 40% had no partner at all.

Among college graduates, the picture is a little different. These young women account for fewer births—college graduates delay having kids generally—and as the number of births goes up, so does the number of marriages. “If marriage retains its place anywhere, it would be among the college graduates,” said Cherlin, “The difference between them and the non-college-educated with regard to the percentage of births within marriage is so striking as to suggest a very different experience of early adulthood.”

Johns Hopkins University

The study points out that unmarried couples have a high break up rate in the first few stressful years after the birth of a child and that this often leads to what’s called “multi-partner fertility” in the academy and “a lot of different baby mamas” in the rest of the world. This kind of family instability, with step-siblings and half siblings and a lot of fleeting parental figures can be tough on both finances and on kids and leads to the calcification of social inequality “The sharp differentiation by education in the transition to adulthood,” says the study, “is another indicator that American society is moving toward two different patterns of family formation and two diverging destinies for children.”

Get our Health Newsletter. Sign up to receive the latest health and science news, plus answers to wellness questions and expert tips. Please enter a valid email address. Sign Up Now Check the box if you do not wish to receive promotional offers via email from TIME. You can unsubscribe at any time. By signing up you are agreeing to our Terms of Use and Privacy Policy . This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply. Thank you! For your security, we've sent a confirmation email to the address you entered. Click the link to confirm your subscription and begin receiving our newsletters. If you don't get the confirmation within 10 minutes, please check your spam folder.

Contact us at letters@time.com.