The modern family size is changing. Four charts and a map show how.

Families are smaller and people are delaying parenthood longer than in previous decades, according to an NBC News analysis of data released this week by the National Center for Health Statistics.

The U.S. teen birth rate reached a record low in 2019, the NCHS report indicates, with fewer than 1.7 births per 100 teen girls ages 15 to 19. While the teen birth rate has dropped significantly since 2007 as part of a long-term downward trend, it remains higher than the rates observed in many other high-income nations.

The overall U.S. fertility rate decreased between 2015 and 2020, additional NCHS data shows, hitting a low of fewer than 6 births per 100 women ages 15 to 44. (The rate subsequently climbed 1% from 2020 to 2021, though the broader trend remains downward.)

The U.S. birth rate — defined as the number of births per 1,000 women — fell between 2018 and 2019 for women in their 20s and early 30s, but rose among women between the ages of 35 and 44, the report found.

The data also showed that from 2015 to 2019, the average age for a woman’s first birth was 24, while the average age for men was 27. In prior years, those averages stood at 23 for women and 25 for men.

Sociologists have identified several factors that may account for these shifts. One is the increased reliability of contraception, while another is that people are marrying later in life, and the majority of births in the U.S. still occur within marriage.

“People are waiting to have kids until they feel ready, they’ve got a good job, they feel mature enough to devote themselves to parenting, they feel like they’re going to have a good partner,” said Karen Benjamin Guzzo, director of the Carolina Population Center at the University of North Carolina at Chapel Hill.

Another potential — though less significant — factor involves medically assisted reproduction, such as in vitro fertilization, which is enabling some individuals to have children at older ages, particularly those with higher incomes or employer-provided benefits.

On average, people in the U.S. are also opting to have fewer children, according to the NCHS report: In 2018, the average woman had approximately one biological child, compared to more than three in 1960.

The average size of a U.S. family — defined as individuals related by birth, marriage, or adoption living in the same household — is now roughly three people, down from nearly four in 1960.

Sociologists suggested that these trends may be largely driven by financial concerns.

“The economic downturn of 2007-2008 really made people go, ‘Wow, having a kid is a big commitment and expensive,'” said Julia McQuillan, a sociology professor at the University of Nebraska-Lincoln.

Guzzo noted that birth rates never fully rebounded following the Great Recession, likely due to issues such as student loan debt, rising housing costs, and a scarcity of full-time employment. Child care expenses and a lack of family leave may also cause people to delay having children or choose to remain childless, she said.

“The United States doesn’t have paid family leave. We have a really shaky child care system, as the pandemic pointed out. We don’t have sick leave for most people. So there’s a lot of things to consider before you decide to have kids in this environment,” Guzzo said.

Fertility rates do vary by region, however: States in the central U.S. report higher rates than other areas of the country.

However, the Supreme Court’s decision to overturn Roe v. Wade, and the subsequent abortion restrictions enacted in many states, could impact future U.S. fertility trends, according to the director of the Maryland Population Research Center.

“It could be that the overturning of Roe v. Wade will act against the continued decline in birth rate,” he said.