For decades, American women have gradually pushed back the age at which they choose to become parents. Now, for the first time, women in their 30s are having more children than those in their 20s, according to preliminary 2016 data released by the Centers for Disease Control and Prevention on Wednesday.

The CDC data shows that the birth rate among women aged 30 to 34 last year just barely surpassed that of women aged 25 to 29, the demographic with the highest birth rate for more than three decades. While women in the early thirties bracket had about 103 births per 100,000 people, the rate for women in their late twenties was 102 per 100,000. On average, the CDC data suggests, women are having their first child at roughly age 28.

That means that first-time mothers are older, broadly speaking, than they were as recently as 2014, when the CDC put the average age of first birth at 26.3. The change is due in large part to a continuing decline in the teen birth rate. Though the CDC has not yet released a full report for 2016, it found a 42 percent drop in the rate of first-time births among teens between 2000 and 2014. But the growing number of first-time parents over 30 are also shifting the CDC’s statistics. In some sectors of American life—especially among more affluent and highly educated women—social norms around age and motherhood have changed with remarkable speed. As one woman who works in media recently wrote about getting pregnant at 27: “The reaction to my pregnancy was neatly split along demographic lines. When I told people over the age of 40 that I was pregnant, they were delighted. People under the age of 40, however, were horrified.”

In many ways, this attitude, and the milestone marked by the new CDC data, represents a gain for women. As Rebecca Traister wrote in her 2016 book All the Single Ladies, the social permission to delay marriage and childbirth—as well the as the biological ability to do so, first through the use of reliable birth control, now with the help of the booming fertility industry—has given women the freedom to define themselves through means other than motherhood. As recently as 1970, the mean age of first birth was just 21, giving most women precious little time to pursue education, adventure, and professional achievement.

However, the rise of older parenthood is also a response to a workplace paradigm that levies penalties on mothers. The U.S. is the only wealthy nation in the world without a federal paid parental leave policy, and childcare in this country is exorbitantly expensive and frighteningly under-regulated. Together, these factors push many women to put off pregnancy in the name of professional success or financial stability.

In her 2012 book Ready: Why Women are Embracing the New Later Motherhood, University of Houston professor Elizabeth Gregory analyzed census data from 2000 and found that delaying motherhood measurably increased women’s earnings. Comparing full-time professional women, all of whom had advanced degrees and were between the ages of 40 and 45, she found that those who’d become parents at 35 were making an average of $50,000 a year more than those who’d had their first child at 20. Even women who’d given birth for the first time at age 30 were making, on average, $16,000 less each year than women who had delayed parenthood by an additional five years. Gregory posited that women who’d become mothers in their mid-to-late 30s had had time to accrue a “shadow benefits system”: clout, expertise, and perhaps bargaining power or flexibility that helped to replace the official benefits that most American workplaces lack.

Older parenthood is not an unalloyed good. In most cases, fertility begins to decline in a woman’s thirties. For those who cross this threshold without realizing it, turning to fertility drugs or technology such as IVF can be miserably difficult and ruinously expensive. And as many people wait longer to have families, family size is slowly shrinking. Currently, the U.S. birthrate still exceeds the replacement rate; falling below it might be desirable from an environmental perspective but disadvantageous from an economic and social one, as conservative pundits love to point out.