Almost half of girls aged 14-21 are embarrassed by their periods, a new survey by Plan International UK found.
As well as being embarrassed by them, one in seven of the 1,000 girls and women interviewed said they didn't know what was happening when they first started their periods.
The charity stressed the importance of boys and girls being taught together about periods at school.
They said many boys want to know more but are stopped by "stigma and taboo".
Katrina, a 16-year-old from Edinburgh, described her puberty education as very limited: "We had one lesson in first year (year 7) where we briefly covered it and there was a lot of giggling and all the boys looked awkward."
She said she and some of her friends hide sanitary items in the rims of their skirts to prevent being asked why they're taking their bags to the bathroom:
"Sometimes it's just easier to come up with an excuse rather than telling someone you're on your period. It's much easier to say you're just not feeling well instead of facing the disgusted looks or the awkward silences."
Her mother Elaine, 52, grew up in Dunfermline, Fife. She said she learned practically nothing about periods at school: "The first real realisation was when girls started asking, 'Have you started yet?' And I wasn't sure what I was supposed to have started so I just said yes."
She said she virtually never discussed them with either of her parents: "A packet of pads appeared in a drawer but nothing was said." She asked her mother to explain what tampons were to her little sister so she could listen in.
'This is what happens every month?!'
"I was embarrassed at school when we had to carry huge pads around in our blazers," she said, adding that there was little choice of product then.
She thinks things have improved: "They're now advertised on TV, comfort is emphasised, there's plenty of choice. My mum's generation, on the other hand, didn't really talk about things like periods - it was just the way it was."
Her mother Veronica, who is now 73, grew up in a family with four girls and very little money: "It was very stressful asking for period products from a financial point of view as my gran was scraping by.
"I was told nothing by my mother. There was almost a shame attached to it."
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Image caption Veronica with her granddaughter Katrina when she was a toddler
Her Catholic school had no sex education. She learned about periods from a book which a girl at school had been given by her parents and they all passed it round for a read: "I thought, 'Oh dear - this is what happens every month?!'"
She said it affected the way she educated her daughter about periods, but that she can now talk quite comfortably about them with her granddaughter Katrina.
"I think it's marvellous that now it can be talked about."
But her granddaughter Katrina said there's still a long way to go: "From a very young age, society constantly dictates to us that periods are something disgusting that we should keep to ourselves.
"I used to hate buying sanitary products and would always avoid eye contact with the cashier and try to pay as quickly as possible. However, recently I realised just how ridiculous that was - half the population menstruates!
"So now I'm not afraid if people see my sanitary products, if they don't like it, that's their problem."