A UN body said that aid agencies and governments must tackle the taboos surrounding menstruation as sidelining the issue undermines the quality of life of women and girls, chiefly in poor nations.
Poor education about menstruation, lack of access to sanitary napkins and painkillers for cramps, and inadequate washing and disposal facilities have a far-reaching impact on schooling, work and health, the Water Supply and Sanitation Collaborative Council said.
"From the moment a girl has her first period, she then menstruates for almost 3,500 days throughout her lifetime, every month," programme manager Archana Patkar said.
"This is the unspoken, silent hygiene and sanitation issue," she told reporters.
A major concern is that a lack of sanitary napkins and washing facilities means girls miss school because of their bleeding, she said.
"This has a huge fall-out, and many implications. Not only for schools, but also in the workplace, in markets," she said.
"This is an issue that cuts across health, education, livelihoods and all development outcomes. It's centre-stage."
The Council pointed to research in India showing that only 12 percent of girls and women have access to and use sanitary napkins, that many have a poor understanding of menstrual hygiene, and that 23 percent of girls drop out of school after puberty.
"There are two billion women worldwide in the menstruating age group, between 12 and 50. At any given moment, 340 million women and girls are menstruating. So the scale of this is pretty huge," Patkar said.
In developed countries women and girls have better access to products and materials, and facilities were also more widely available, she said.
She criticised beliefs in some societies -- notably male-dominated ones -- that menstruation is "impure".
"Menstruation is a biological phenomenon which is responsible for future generations. We wouldn't be here without it. So it's really strange that we have all this silence, shame, secrecy and taboos around it," she said.
"This has huge psychosocial implications for a young girl which carry right through to old age. It's part of the disempowerment and disenfranchisement of women and girls."
The Council groups UN agencies, governments and professionals, and focuses on improving the lot of the 2.5 billion people worldwide without access to basic sanitation.