Ever wondered what those icky-sticky, thick clumps of blood in your period are? They're known as menstrual clots, and they're formed from a mixture of blood cells, tissue from the lining of the womb, and proteins from your blood.
Firstly, it's important to be aware that everyone has menstrual clots to some extent, and it's not necessarily a sign that anything's wrong.
"Usually clots occur when the flow is a little bit heavier - generally the first two days of your period," Dr Vanessa Mackay, spokesperson for the Royal College of Obstetricians and Gynaecologists (RCOG) explains. "When you're bleeding heavily, the blood pools inside your vagina and it clots, much as it would do if you were bleeding elsewhere."
When to worry
Like many things to do with gynaecological health, what's considered 'normal' varies from one person to the next - so you might regularly experience heavy bleeding with clots, while your best friend might have a much lighter flow and rarely ever get any clotting.
The key thing here is knowing what's normal for you, and that you only need to contact your doctor about menstrual clots if you're worried about them in any way.
"If it changes significantly from what's normal for you - so you suddenly get much bigger or more frequent clots - that might be a sign of something else," Mackay says.
"A way to check for that would be if you're having to change your sanitary product every hour or two, needing to wear tampons and pads together, or if the clots are larger than, say, a ten pence piece," she explains. "Anything that suggests your period's got a lot heavier than normal might be an indication you need to see your doctor."
What it could mean
Even if you've noticed a change, rest assured that there's still not necessarily any reason to panic.
"For most women, about 70%, there is no underlying cause or reason for heavier bleeding and bigger or more frequent menstrual clots," Mackay says.
There are, however, certain conditions that could be causing it, so it's important to see your doctor and get it checked, so that these can be ruled out.
Uterine fibroids are non-cancerous growths of the womb, which can cause heavy, painful periods.
Pelvic inflammatory disease (PID)
Infections like pelvic inflammatory disease can also cause abnormal bleeding.
Endometrial polyps are non-cancerous growths in the lining of the womb, and can also cause heavier periods.
Blood clotting disorders
A blood clotting disorder might also cause large and frequent menstrual clots.
Heavy bleeding and clotting could, of course, also be a sign of something more sinister, like womb or uterine cancer - although, as Mackay points out: "That's very rare before the menopause."
Other factors to consider
The pill is traditionally taken for 21 days, followed by a seven-day break, during which time you have what's known as a 'withdrawal bleed', similar to a period. Alternatively though, you can choose to take the pill continuously without a break, or take it for several months at a time followed by a break. If you are taking several packs back-to-back, followed by a pill break, this may cause your withdrawal bleed to be heavier, with more clots, than usual - or it may have the opposite effect.
"It's difficult because there are so many variables - it's hard to say exactly what's normal or abnormal, or what to expect," Mackay says. "Your weight has an influence, which hormones you're taking, where you are in your cycle, all of these things."
Finally, any unscheduled heavy bleeding and clotting, when you're not expecting your period, could signal a very early miscarriage.
"If generally you have regular periods, and suddenly you have a very unscheduled bleed that's quite heavy, you should take a pregnancy test. If the test is positive, that would alert you to the fact that perhaps it's a very early pregnancy loss rather than just a heavy period," Mackay explains.