Robe- and mortar-bedecked students during a recent graduation at Northwestern University in Evanston, Ill. (Photo by John Kelly/The Washington Post)

There’s a lot that many college freshman don’t know about sex. And when they don’t know something, they often stay in the dark, too afraid or embarrassed to ask about it.

So Northwestern University is opening up an online class to help the teenagers — and the curious, everywhere — learn more about reproduction. It’s called “Sex 101.”

“Having sex is not the same thing as knowing how it all works,” said Teresa Woodruff, the Northwestern University obstetrics and gynecology professor at the school of medicine who created the new course. “This is everything first-year students need to know about sex and reproduction, and didn’t know to ask.”

The class begins Sept. 28 and is open to Northwestern students — and the general public — through the “massive open online course” platform Coursera. Through animated videos, the course will cover the basics of sexual body parts, reproduction and sexually transmitted diseases. Northwestern hopes that the course will supplement sex ed classes the students experienced in high school before landing on campus this fall.

[To address college sexual assault, some say kids need more sex education before they get there]

Because those high school courses vary widely — and because fewer than half of the states require that it be taught — students arrive on college campuses with a range of prior knowledge, with some of them knowing little to nothing.

From a safety perspective, experts worry that college freshmen arrive on campus with vastly different concepts of what constitutes consensual sex and gaps in their knowledge that can leave them vulnerable to assault. A recent Association of American Universities survey and a recent poll by The Washington Post and Kaiser Family Foundation found that approximately one in five women experienced unwanted sexual contact while in college. The Washington Post-Kaiser poll also showed that two people can interpret the same behavior differently: More than 40 percent of students said that nodding in agreement established consent, for example, and more than 40 percent said it did not.

[What a massive sexual assault survey found at 27 top U.S. universities]

Additional sex ed could help fix such critical misunderstandings, both about consent and about the mechanics and implications of sexual relationships.

Woodruff noted that many male freshmen don’t know, for example, that alcohol can significantly diminish their ability to maintain an erection, which “can be perplexing to a young man if he’s not aware of the cause of this issue,” the Northwestern statement explained.

Young women, when they first arrive in dorms, can experience profound fluctuations in their menstrual cycles, which can range anywhere from 14 to 40 days because of the sudden change in their environment, Woodruff said. That can also affect fertility, meaning they could be more likely to get pregnant at certain times without realizing it.

Not being aware of such information about their own bodies can lead to unintended consequences, such as unplanned pregnancies or sexually transmitted diseases. Woodruff said, for example, that all students should know that one in four college students has an STD.