Periods are a part of life for much of the population, but talking about them openly can be difficult for some people.
But now they're a topic of government debate, and "period poverty" initiatives keep making headlines.
Conversations are opening up, and new products are on the market.
How much do women and girls spend on period products?
In the UK, it's estimated that the period product market was worth £265.8m ($350.4m) in 2017, according to retail researchers Mintel.
Tampon, menstrual pad and panty liner purchases made up most of the spend. But consumers have been spending less, with sales dropping by £5.7m since 2016.
Hera Crossan, research analyst at Mintel, said that the drop was down to supermarkets cutting their prices to attract more shoppers, rather than people cutting down on what they are buying.
According to the charity Bloody Good Period, the average lifetime spend on period products adds up to around £4,800.
Are reusable products becoming more popular?
While tampons and pads are popular options for managing periods, Instagram and Facebook users might have noticed more adverts for reusable products popping up on their feeds.
Reusable period products can include tampon applicators, menstrual cups or cloth pads.
Period-proof underwear typically has layers of cotton and waterproof material, so they can be worn instead of (or with) other period products.
The number of Google searches for reusable products has increased in recent years, with the menstrual cup proving the most popular.
The search engine uses a number out of 100 to represent interest in a search term. In 2013, the search "menstrual cup" had a popularity score of 21, but it has increased to 83 only five years later.
Most searches came from Australia, while the United States had the most for reusable menstrual pads.
Big retailers like Walmart in the US and Boots in the UK now stock menstrual cups, and some companies have reported that sales have been growing at double digit rates over the last 10 years.
What's behind the change?
The impact of plastic pollution on the environment has recently been making headlines. In the UK, the government has announced that it would launch a consultation on plastic pollution, potentially increasing the plastic bag fee in England to 10p.
Analysts have said that growing concern over the environment is one of the factors behind the increased interest in reusable period products.
Many tampons and pads are often wrapped in plastic, and most discarded products end up in landfill.
In 2017 the Marine Conservation Society also reported a large number of period products washed up on British beaches - including pads and single-use plastic applicators.
As awareness of plastic pollution increases, plastic-free periods are being popularised on social media. Instagrammers and YouTubers are setting up channels to promote "eco-friendly" periods.
It's also thought that women are turning to reusable products in order to save money.
What is the tampon tax?
The tampon tax refers to the revenue earned by a government when VAT (or sales tax) is applied to the sale of period products.
Tampons and other period products are currently classed as luxury or non-essential items by the European Commission, so they can be taxed in the UK.
Period poverty campaigners like Amika George and Freedom4Girls have claimed that children from low-income families have been unable to afford menstrual products, and have missed school as a result.
Australian federal and state governments agreed this October to remove their levy after its introduction in 2000.
India also recently scrapped its 12% tax on period products following campaigning by activists, while Canada did the same in 2015.
The European Commission is currently reviewing changes to VAT rules which would allow governments to scrap the minimum 5% rate on period products.
In the meantime, the UK government has pledged to invest the money raised by the tax in projects that benefit women and girls. Some supermarket chains have also been covering the 5% tax as a goodwill gesture for customers ahead of it being abolished in the UK.
Period products in Ireland were already zero-rated before the EU legislation was introduced.
Illustration by Katie Horwich.