The pain is caused by the womb tightening and relaxing to encourage its lining to shed - a process that happens each month as part of a woman’s menstrual cycle. Picture: Flickr.com
Image: supplied
Finding what appears to be a piece of tissue shaped like the inside of your uterus can be terrifying, especially if you've never heard of a decidual cast.
Although rare, medical experts say the phenomenon is becoming more widely recognised, partly because more people are sharing their experiences and because of greater awareness of hormonal contraceptives.
A decidual cast occurs when the entire lining of the uterus is shed in one intact piece rather than breaking apart during menstruation. The result is a piece of tissue that can resemble the shape of the uterine cavity and is often accompanied by severe cramping and pelvic pain.
While it can be alarming, a decidual cast is usually not dangerous. However, experts advise anyone experiencing one for the first time to seek medical attention to rule out other conditions, including miscarriage or ectopic pregnancy.
The uterus is designed to build up and shed its lining every month under the influence of hormones, particularly oestrogen and progesterone.
Normally, as progesterone levels fall before a period, the uterine lining breaks down into small fragments that leave the body as menstrual blood.
However, in rare cases, a sudden surge or drop in progesterone can cause the lining to become unusually thick and remain intact. Instead of disintegrating, it peels away from the wall of the uterus in a single piece, creating what is known as a decidual cast.
This hormonal shift is why the condition has been linked to certain forms of hormonal contraception, especially those that contain only progesterone.
Although uncommon, decidual casts have been reported in people using the progesterone-only pill, contraceptive implant, injectable contraception and even after abruptly stopping hormonal birth control.
As long-acting reversible contraceptives continue to gain popularity, more women are sharing their experiences online after unexpectedly passing a decidual cast.
Videos on TikTok and discussions across social media have drawn attention to the condition, with many users describing intense pain before passing tissue that looked unlike a normal menstrual clot.
The growing awareness has encouraged healthcare professionals to explain that while the experience can be frightening, it does not necessarily indicate a serious underlying problem.
When should you see a doctor?
Doctors recommend seeking medical advice if you experience severe pelvic pain, heavy bleeding, fever or if there is any possibility you could be pregnant.
Because a decidual cast can resemble tissue passed during a miscarriage, a healthcare provider may recommend a pregnancy test, pelvic examination or ultrasound to determine the cause.
If you recently started, stopped or changed hormonal contraception, letting your doctor know can also help identify whether hormone fluctuations played a role.
Although a decidual cast is rare, understanding how hormonal contraception influences the menstrual cycle can help demystify one of the body's more surprising and often unsettling responses to changing progesterone levels.
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