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May 25, 2026
Perimenopause or Stress? How to Tell the Difference
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Decidual casts - what they are, why they happen, and why most of us have never heard of them.
Here's something wild: you can know a lot about reproductive health, follow every cycle account on the internet, have lived in your body for decades, and still never have heard of a decidual cast. We hadn't either, until recently. So if this is your first time, welcome to the club.
A decidual cast is a piece of the uterine lining that comes out during a period in one solid, mostly-intact piece, sometimes shaped like a small mold of the inside of the uterus. It's rare, it's not dangerous, and it's terrifying the first time it happens. Almost nobody warns you it's possible.
In a typical period, the lining of the uterus breaks down a little at a time over a few days. It comes out as the mix of blood, fluid, and small bits of tissue you're used to seeing.
With a decidual cast, the lining comes out in one piece instead. The "decidual" part of the name just means the lining had thickened up the way it does in early pregnancy, but no pregnancy was actually there.
A decidual cast looks like real tissue: usually pink or red, soft, fleshy, sometimes the rough shape of the inside of the uterus. That's what makes it so different from a blood clot. Clots are dark and jelly-like. A cast is recognizably tissue. If you take it to a doctor, a lab can confirm what it is.
According to a 2026 scoping review in the International Journal of Gynaecology and Obstetrics, the most comprehensive look at this topic to date, mapping 113 publications, the proposed causes fall into a few buckets:
Hormonal birth control or hormone medications. This is the biggest one. Many reported cases involve people on combined pills (the regular pill, with estrogen and progestin), progestin-only options (mini-pills, the hormonal IUD, the implant, the shot), the ring, or hormonal treatment for endometriosis. The common thread: progesterone, one of the hormones in almost every kind of hormonal birth control. When the level of progesterone in your system drops suddenly, a missed dose, switching methods, the end of a pill pack, the lining can come out all at once instead of gradually.
Your own hormones doing the same thing. It can also happen during...
With a background in nursing and a genuine passion for care, Jessie supports myStoria members as part of the Concierge team.
