And there’s a natural follow-on trend to notice: As the dark gray line in the above graph shows, moms living with their own kids represent a shrinking share of the population too. This makes intuitive sense: Minors are much more likely to live with their parents than are the group of people with living mothers generally, and so if the fertility rate falls, so would the percentage of the population who are moms with kids in the house.

Not only are moms making up less of the population, but their characteristics are changing too, and in a way that might be linked to their proportional decline. Moms today tend to be older than in the past. Just looking at recent years, the change in age-specific birth rates has been drastic.

Number of Births Per 1,000 Women, by Age Group

(The Atlantic | Data: CDC)

In just the past few years, the peak childbearing age range for American women has advanced from 25-to-29 to 30-to-34. Meanwhile, childbearing among women under 20 has fallen by half or more, while childbearing among women 35 and older is rising.

One positive consequence of this age shift is that a larger proportion of new mothers are economically prepared to raise children. Less positively, however, many women find that, as they age, they can’t have as many kids as they would like. Plus, having children later in life can increase the risk of post-partum health complications. These finer points aside, one major consequence of the older-moms trend is that fewer years of a woman’s life are spent as a mother. This means that, at any given time, a larger share of women, and thus of the whole population, will report not having children in government surveys. In other words, later motherhood means less motherhood.

What is at the root of later-in-life births? Many people, especially many well educated people, are delaying marriage (for many reasons); even if people do eventually get married, they spend less of their life in a marriage, which tends to mean fewer babies. Furthermore, the wider use of contraception and developments in sex ed are contributing to much lower rates of teen pregnancy. It’s also possible that increased use of social media and pornography is leading to diminished frequency of unprotected, high-pregnancy-risk sex among teens: The idea is that with less face-to-face time, there’s a reduced supply of opportunities for spontaneous sex, and with more to do on phones, there’s also reduced demand for it.

On the higher end of the age distribution, the biological potential for childbearing is improving thanks to a panoply of fertility-enhancing medications, reproduction-assisting technology, better neonatal and OB/GYN care, and improved maternal education. As a result, women are having more kids later, although it is extremely unlikely that these later-in-life babies will offset the reduced earlier-in-life fertility.