Cohabiting parents Amanda Leigh Pulte & Matthew Gage with their daughter, Zoey, at their home in Austin, Texas. (Eric Gay/AP)

) -- More than half of millennial women aged 26 to 31 who have babies are either unmarried or single instead of part of a married couple, according to a recent study from Johns Hopkins University.

Fifty-seven percent of children born to millennial women are born out of wedlock, and 63 percent are born to women lacking a college education, according to the Hopkins study, entitled “Changing Fertility Regimes and the Transition to Adulthood.”

In fact, the only group that primarily follows a more traditional marriage-before-children pattern are college graduates, 90 percent of whom have children within marriage, the study found.

“It is now unusual for non-college graduates who have children in their teens and twenties to have all of them within marriage,” the study's authors said.

Of the 57 percent of births outside of marriage, 26 percent were to cohabiting mothers and 31 percent were to women who were not in stable relationships. Among non-college grads, 29 percent of the 63 percent of births outside of marriage were to cohabiting mothers and 34 percent to unattached mothers.

The trend toward out-of-wedlock births is more pronounced among those with only a high school diploma or some college. In the past, such births primarily occurred among high school dropouts and minorities.

In addition, millennials are also pushing marriage away from their young adult years and into their late twenties and thirties: “The lofty place that marriage once held among the markers of adulthood is in serious question among early adults,” the study says.

One author of the study, Elizabeth Talbert, believes that the less educated view marriage as financially out of reach, leading to cohabitation as a substitute. “Structurally and economically, it's just harder for people without good, stable jobs to attain the kind of life they want to have in a marriage, and marriage is often delayed.”

However this theory does not explain why lower- and middle -class couples also choose to have children while delaying marriage.

“Most women without a college degree continue to experience ‘love and babies’ in their early twenties, just without the benefit of marriage,” according to Knot Yet, a similar study about the trend to delay or forgo marriage. “A majority of first children born to parents under thirty are born outside of marriage and exposed to the economic, social, and familial fallout associated with a nonmarital birth.”

(AP photo)

Knot Yet speculates about why poorer couples tend to have children earlier, noting that unlike female college graduates who may want to attain certain career goals before having kids, less-educated women “do not have access to the kinds of jobs that would propel them into a comfortable middle-class lifestyle.”

When marriage and kids after a career no longer seems attainable or even desirable, “poor and (increasingly) Middle American women turn instead to the traditional source of young-adult female identity—motherhood—for meaning and satisfaction,” the website says. "They end up setting a lower bar for deciding on the father of their child than for choosing a husband."

Knot Yet also points out that the culture no longer sees marriage and children as a package deal, making it more societally acceptable to live a married life without kids, or cohabit with children. Especially among the middle and lower classes, there’s less of a stigma to non-marital childbearing and the high rate of divorce no longer makes marriage seem like a safer option than cohabitation.

However, according to the Hopkins study, cohabitation frequently provides a more unstable environment for raising children, especially among young parents: “We know that early non-marital childbearing is associated with union dissolution because of the high break-up rates among unmarried parents in the first several years after a birth.”

Talbert says that birth outside of marriage does not always lead to fatherlessness, which she says is “definitely a worrisome phenomenon.” Instead, it often leads to a more complicated family structure in which “a child is often not ‘fatherless,’ but instead might have a complex relationship with her father that includes visitation, extended family, and step siblings.”

Having a father who lives outside the home can be harmful to a child’s development; The National Fatherhood Initiative says that such children are more likely to use drugs, be incarcerated and have emotional or behavioral problems than if they were raised with both their biological mother and father present.

The current percentage of out-of-wedlock births is a 46 percent increase from the 11 percent of non-marital births reported in 1970, according to Child Trends.