Boys and girls should be taught about periods at school, a charity says.
Plan International UK says not talking about it can be damaging and wants it to be discussed in class.
The charity has carried out a survey of 1,000 girls aged 14-21, which suggests almost half are embarrassed by periods.
"I think there is a stigma and taboo around periods," says Kerry Smith, head of girls' rights at Plan International UK. "Girls and boys aren't being told about periods enough."
As well as being embarrassed by them, one in seven of the girls and women interviewed said they did not know what was happening when they first started their period.
Nina is 18 and a student. She was 12 when she first got her period.
"I just looked down and there was blood. I was freaking out."
Her mum gave her a sanitary towel but did not say much else.
"I put on the pad and thought, 'What if I'm dying?'"
Innes, 18, says she struggled talking to her male friends.
"I was on my period and wanted to buy a sanitary towel and the boys found out. They started saying, 'We can smell the iron in the air.' I was dying in the corner."
But Clare, an 18-year-old university student, says she talks to her brother about it.
"They're not going to die if you tell them about your period," she says. "Just get over it."
The survey suggests only 24% of girls feel comfortable discussing their period with their male friends.
Nidar, 19, says he does not want to know about periods.
"Period is a natural thing, it's not your fault you have it, but you don't have to mention it", he says. "If I have a flu, I'm not going to tell you I have a flu today."
Kerry Smith from children's charity Plan International UK says boys have told her they want to know more.
"We do think boys and girls should be taught together and boys have told us they don't think it's right they don't know anything about menstruation."
The charity says not talking about it can be damaging and wants boys and girls to learn more about periods throughout secondary school.
A spokesperson for the Department of Education in England said: "Schools can already teach about menstruation through an inclusive sex and relationships education programme - it is also part of the national curriculum for science.
"Schools are free to tailor lessons to suit the needs of their pupils, drawing on resources from expert organisations to ensure pupils are taught about menstruation and to support girls to feel positive about their bodies."