Many U.S. states don’t require schools to teach sex education. And those that do often emphasize abstinence.

In Denmark, falling birth rates have led policymakers to spice up the sex-ed curriculum. Rather than focus just on preventing pregnancy, teachers are also warning teens about the risks of waiting too long to have kids.

Average Age of Women at Childbirth in Denmark

Eurostat / Quartz

“Young people want between two and three children and they are having between one and two,” says Bjarne Christensen, secretary-general of Sex and Society, a nonprofit group that provides much of Denmark’s sex-ed teaching materials. “Most people are getting fewer than they thought they could have, and fewer than they wanted.”

Across Europe, aging populations are putting pressure on the continent’s generous social services. This also plays a role in sex-ed revamps like the one in Denmark, which launched last year.

“We need to educate men that if they want children, they need to get on with it while their partner still has time,” Lone Schmidt, a public health professor at Copenhagen University, told The Guardian.

Sex and Society suggests that teachers ask students to draw up the pros and cons of being early parents, or setting out their ideal vision for a good family life. How many kids does that entail? When should the students have them? What “actions” need to be taken to make that happen?