As many as 139,000 women in Scotland could be suffering from endometriosis - a condition that causes painful, heavy periods and can lead to infertility.
One in 10 women of reproductive age is likely to have the condition, according to charity Endometriosis UK.
And many of them will not even be aware there is anything wrong, experts say.
The NHS says girls typically start their periods at the age of 12 and the average age of menopause is 51 years old.
That means there are 1.4 million women and girls of reproductive age in Scotland, according to the latest population statistics.
If one in 10 suffer from endometriosis, that would make a total of more than 139,000 with the condition.
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Across Glasgow and the surrounding areas, there could be around 36,130 with the condition.
There could be around 18,254 women and girls in Glasgow city suffering from endometriosis out of a population of 182,537.
Elsewhere, there are an estimated 8906 suffferers in North Lanarkshire, 7900 in South Lanarkshire, 2299 in East Renfrewshire, 2279 in West Dunbartonshire and 4502 in Renfrewshire. See the table at the bottom.
Endometriosis is a condition where cells like those in the lining of the womb are found elsewhere in the body.
Every month the cells react in the same way as those in the womb by building up and breaking down and bleeding.
Unlike the cells in the womb that leave the body as a period, this blood has no way to escape.
That leads to extremely painful, heavy periods and in some cases can also cause infertility, fatigue and bowel and bladder problems because of inflammation and scar tissue.
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Campaigners say that not enough is known about the illness and that, despite more women sharing their struggles with the illness in recent years, more needs to be done to raise awareness of the condition which takes an average 7.5 years to diagnose.
A spokesperson from Endometriosis UK said: “As it’s a hidden condition, it can be hard for those with the disease to get others to understand or believe the level of pain they are suffering from.
“There needs to be more education and awareness raising so people recognise the symptoms of the disease and can seek help, and more research into endometriosis to develop effective diagnosis and treatments.
“Historically women’s menstrual wellbeing is not an area that has had much research investment compared to many others, and we need to see this rectified.
“Unfortunately, there is limited data collected consistently on all aspects of endometriosis.
“During the potentially long diagnosis time, endometriosis may not be suspected and other conditions tested for.
“If there is not accurate and consistent recording in the same way for those diagnosed, we will not get good data.
“And if the impact of the disease is under recorded, there will not be enough resource planned to support those with the disease.
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“Women’s pelvic pain needs to be taken seriously.
“All too often, women are told their pain is “normal” or “in their head” and it is not taken seriously.
“This needs to change in order to reduce diagnosis time of endometriosis.
“Delayed diagnosis and treatment of endometriosis can have a huge impact on a woman’s quality of life –education, work, relationships, and personal life –and may result in the disease progressing.”
There are around 16.9 women and girls of reproductive age across the UK - meaning as many as 1.7 million could have endometriosis.
It is worth noting that these figures should be treated as estimates because some girls may start their periods before or after the age of 12 and some women will go through the menopause before or after the age of 51.
The NHS was contacted for comment.
