Total fertility rates fell across the U.S. during the last decade, even as the gap in rates widened between rural and metropolitan counties, according to a new report from the Centers for Disease Control and Prevention.

Total fertility rates – representing the estimated number of lifetime births expected per 1,000 women – fell 18 percent in large metro counties and 16 percent in small or medium metro counties between 2007 and 2017, the report said. Yet while the rate in rural counties fell 12 percent in that time frame, the decline slowed after 2011, resulting in a 14 percent gap between total fertility rates in rural counties and large metro counties last year. The gap was just 5 percent in 2007.

Rural areas have been associated with higher fertility rates and worse birth outcomes than urban and suburban areas, the report said. The total fertility rate was 1,950 births per 1,000 women in rural counties in 2017, compared with 1,778 in small or medium metro counties and 1,712 in large metro counties, according to the CDC.

The age at which women became mothers also rose during the last decade, though gaps remain between rural and urban areas. In 2017, women had their first children, on average, at 27.7 in large metro counties, at 25.8 in small and medium metro counties, and at 24.5 in rural counties.

“Back to the 1950s and 1960s, there were these differences between rural and urban areas, but there was also this expectation that at some point, these differences would disappear,” says Danielle Ely, a health statistician at the CDC’s National Center for Health Statistics.

“What we’re seeing (instead) is that even as patterns are the same in (urban and rural) areas, there still is this difference,” Ely says. “It’s indicating that there might be different needs in rural counties than large metro counties in some ways … in terms of medical care for mothers and for infants.”

The significance of these gaps between urban and rural counties will vary by community, Ely says. But they are important to consider as rural areas lose access to critical health care services. For example, the share of rural counties with hospital obstetric services fell from 54 percent in 2004 to 46 percent in 2014, according to a policy brief from the University of Minnesota Rural Health Research Center .