Serena McNeal smiles as she cuddles her 5-month-old son, Seamus, who wriggles happily and grins right back at her.

McNeal, 31, is among a rising tide of older single women who are having babies: While the birth rate is falling across the United States, and births to unwed teens have plummeted, more older single women are having babies.

Pennsylvania's overall birth rate dropped by 15.32 percent between 1991 and 2011, according to an analysis of data released by the state Department of Health. Unwed teen births fell by 31.85 percent.

But births to unmarried women older than 20 rose by 34.66 percent, and births to unmarried women of all ages rose by 15.08 percent in the state over that same period.

Nationally, 35.7 percent of births in 2011 were to unmarried women; in Pennsylvania, the rate was 40.4 percent, according to the U.S. Census.

Locally, between 1991 and 2011, the unwed birth rate rose by 34.5 percent in Carbon County; by 66.69 percent in Lehigh County; by 55.07 in Northampton County; by 126.53 percent in Monroe County, and by 46.46 percent in Schuylkill County.

All the single ladies, having babies

A 1995 report to Congress on out-of-wedlock childbearing by the U.S. Department of Health and Human Services outlines some contributing factors.

"The dramatic increase in unmarried childbearing in the United States reflects changes in marital behavior as much or more than changes in fertility behavior. Americans are not having more babies; they're having fewer marriages," the reports says.

People begin having sex at younger ages, but are delaying marriage or forgoing it altogether. High divorce rates and more people living together without marriage also contribute to the unwed baby boom.

"Most nonmarital births are unintended, as parents are unable to obtain, do not choose, or fail to use effective contraception on a regular basis, the report says.

The reasons more older women are having babies vary. According to the U.S. Census, they include changes in norms of sexual behavior and how families are formed.

August Aldebot-Green of Child Trends believes the reasons for the jump in unwed motherhood are complex.

"Most immediately, however, the increase can be linked to the decline and delay in marriage among men and women. But it is critical to remember that many of these 'nonmarital' births do in fact occur in two-parent households, to parents who are cohabiting. Estimates suggest that over one-half of nonmarital births occur to cohabiting parents. However, cohabiting unions tend to be less stable than marriage," he says.

Rise of the

older new mom

In the early 1990s, teens accounted for the most unwed births. But by 2008, that trend had reversed, with women 35 and older having the most children out of wedlock. Since the early 1990s, women in their 20s who have at least some college education account for the fastest-growing birth rate, according to government data analyzed by Child Trends, a Bethesda, Maryland, based research group.

It's a description that fits McNeal almost to a T.

McNeal, who also has an 11-year-old son, Logan, feels that the extra years give her an advantage.

Seamus was a planned baby. Logan was a happy surprise for McNeal, then 19 years old and single.

"You're more aware of consequences. when you have a child when you're over 30, it's not something that 'just happens.' We planned to have Seamus. We talked about and thought about all the consequence that entailed," she says.

"You're aware of the commitment. You're not a child. Even if you are a wise child, you don't realize what it is to commit your life to another person that way," McNeal says.

Being an older single mother brings challenges, too. Money is a big concern. If a single mother works, that takes precious time away from her baby. But if she opts to stay home, money is tight.

"I've chosen not to give up that time, so I will do whatever is necessary to save money," McNeal says.

She is vegan, which helps on grocery costs. Many of Seamus' clothes are hand-me-downs, thrift shop finds or recycled. One recent day he wore cozy soft pants made from one of McNeal's old sweaters. She shops at farmers' markets or in bulk online. The only new clothes she buys for her family are undergarments.

But while McNeal is confident she will nurture and guide young Seamus to a happy, productive life and provide for his needs, experts say not all children of unmarried mothers will fare as well.

"Children who are born to unmarried parents are more likely to live in poverty and to have poor developmental outcomes," Rachel M. Shattuck and Rose M. Kreider wrote in a May 2013 American Community Survey report.

According to Child Trends, "children born to unmarried mothers tend to have access to fewer socioeconomic resources than those born to married mothers. This matters because these resources are linked to the overall health and well-being of children."

Fewer teen mothers

The numbers of births to teenagers has dropped more than 50 percent nationwide since the early 1990s, according to the National Campaign to Prevent Teen and Unwanted Pregnancy.

In Pennsylvania, the rate has dropped

"Make no mistake. The credit for this great good news goes to teens themselves," says NCPTUP's chief program officer Bill Albert. "More are delaying sex, they are reporting fewer sexual partners, and they are using contraception more consistently and carefully. In short, the magic formula of less sex and more contraception is behind the historic declines in teen pregnancy.

"Thanks in part to a robust federal investment in proven approaches, more communities are using effective, research-based programs that actually move the behavior needle," Albert says.

Other contributing factors may be surprising.

"There is now credible evidence that MTV's wildly popular shows "16 and Pregnant" and "Teen Mom" have played a role in the nation's progress in preventing teen pregnancy teens view these shows as more sobering than salacious," he says. "There has also been a substantial amount of time, attention and action on this issue at the state level."

The bottom line is, Aldebot-Green says, that "many children will continue to be born outside of marriage. To promote children's well-being, healthy relationships among all family members, including those living outside the household (nonresidential parents or grandparents), should be supported, and financial supports from both parents should be available to each child. In addition, efforts to help couples prevent unintended pregnancies continue to be critical, and these efforts need to recognize that many of these couples with unintended births are not teens but young adults.