“… First comes love, then comes marriage, then comes the baby in the baby carriage.” A new Census Bureau report shows that this childhood rhyme is increasingly no longer holding true in the U.S.

For the first time, the number of first-born babies to a married mother and father has fallen below 60 percent. Meanwhile, child birth among cohabitating parents has risen to more than one in five.

Birth statistics have changed significantly over the past four decades. According to a new report “Fertility of Women in the United States: 2012” which examines Census data on fertility and child birth, the average number of children born has dropped from more than three children per woman in 1976 to about two children per woman in 2012. Recent years have also seen declines in adolescent childbearing, which we might applaud, but increases in non-marital births, a more troublesome trend.

A woman having her first baby is still more likely to be married but that likelihood is falling. The number of unwed mothers is rising whether she is with a “stable” partner or on her own. Among young first-time mothers, the growth in cohabitation roughly parallels the decline in marriage.

The Washington Times reports more findings from the report:

During the 1990s, around 70 percent of first-time mothers of all ages were married, with about 18 percent single and the rest cohabiting, the report said.

By the years 2005-2012, however, data showed that a significantly smaller majority of new mothers — 55 percent — were married, while 25 percent were cohabiting and 20 percent were single and without a steady partner.

Moreover, the new data show that for the youngest mothers, norms surrounding marriage and childbearing have flipped.

Before 1990, more than 60 percent of first-time mothers aged 23 or younger were married, with smaller cohorts either cohabiting or single.

But after 2005, only 24 percent of these young women were married when they had their first child. Instead, they were more likely to be either cohabiting (38 percent) or not in any defined relationship at all (38 percent).

The phenomenon of children born out of wedlock is a troublesome trend for society. As the report indicates, a women’s child bearing is related to her rates of employment, educational attainment, and economic well-being. Children are also affected as the circumstance into which they are born is connected to that child’s later outcomes, including the likelihood of living in a single-parent household and academic achievement.

Poor, unwed mothers have been the face of welfare and food stamps for decades. Welfare reform in the 1990s went a long way in moving single moms from government dependency, but public and private efforts can only go so far in helping families when there is just one parent. Two married parents provide a level of security and stability that is difficult for one parent. The addition of a random partner not living in the household doesn’t necessarily go very far either (whether that “partner” is a person or government aid).

Young people are choosing to delay or avoid marriage but not postpone child bearing which may be due to a number of factors. As this trend continues, researchers will be able to track the success of children raised by cohabitating parents.

We do know that single parenthood is difficult -even for mothers who are financially secure when they learn that a bundle of joy is on the way. Many single mothers live on the financial edge (getting by one paycheck at a time). Even those single parents are not living in poverty can be at risk following the loss of a job, downturn in the economy, or unforeseen illness.

As the Millennial generation continues to enter the child bearing phases, we’ll see if these trends will continue and the impacts on children.