Nearly two in three young women (64%) knew very little or nothing at all about periods until their own menstrual cycle started, a poll suggests.
More than a third (34%) of boys and young men are still in the dark on the subject, the survey found.
The findings are part of a WaterAid campaign to get young people to talk openly about periods in an attempt to combat taboos around the subject.
The charity has teamed up with children’s author Dame Jacqueline Wilson and Doctor Who actor Jodie Whittaker to launch an animation to help encourage children, teachers and parents to have more conversations about periods.
It comes as teaching about menstruation will become compulsory in all state schools in England from this academic year, 2020-21, as part of reforms to relationships, sex and health education.
A survey of more than 1,000 14 to 21-year-olds from across the UK found that nearly a quarter (23%) of girls said they felt scared when they started their period as they did not know what was happening.
Nearly one in four respondents (24%) said they would describe periods as embarrassing and a fifth said they would describe them as gross.
One in seven (14%) of the young women and girls surveyed said they have been teased about their periods, and a quarter of the boys and young men have joked with their friends about periods.
Girls and young women reported that although they have concerns about their periods, they would not always feel comfortable discussing their worries.
Nearly four in five (78%) of the young people surveyed said they felt that periods should be discussed more openly with everyone.
The animation P.E.riod Excuses – written by Dame Jacqueline, narrated by Whittaker and illustrated by Nick Sharratt – features girls discussing their own periods and the issues that surround them.
Dame Jacqueline said: “My stories deal with the real-life situations children and young people face, and this short story is no exception.
“I wrote P.E.riod Excuses for WaterAid to help get children talking openly about periods, because such a normal, natural part of life shouldn’t be a taboo subject.
“I believe storytelling – whether that’s speaking, writing, film making, drawing, or anything else – is one of the best ways to address topics that might seem difficult to talk about.”
Whittaker said: “I like to show girls that they don’t have to be anyone’s sidekick and I think ending the embarrassment about the normal things girls face, like periods, is an important part of that.
“We can only do this by encouraging more open conversations about menstruation, with boys as well as girls, and both at school and at home.
“This can have a powerful knock-on effect, helping ensure the needs of women and girls are not overlooked.”