WTF Is A “Decidual Cast”? Decoding This Mysterious Period Symptom

By now, you might think you're aware of every period symptom out there — that is, until you look down and spot something you've never seen before. Take, for example, a decidual cast, which has gone viral on TikTok with plenty of people wondering what in the heck it actually is.

If it's ever happened to you, you've likely been confused and perhaps even alarmed, wondering if you should seek medical attention. Fear not, friend. Two OB-GYNs are here to explain what this menstrual symptom is and why it happens.

What is a decidual cast?

Simply put, "a decidual cast happens when you shed the entire lining of your uterus in one piece," as Nita Landry, MD, FACOG, a board-certified OB-GYN and author of Dr. Nita's Crash Course for Women: Better Sex, Better Health, Better You, tells Scary Mommy. So if you look down and notice a triangular-shaped piece of tissue, you now know the name for it.

Kim Langdon, MD, an OB-GYN with Medzino, notes that this "pink or red fleshy tissue" (the thick mucus lining of the uterus, aka the decidua) will typically be the size of your palm, and you might think you are miscarrying — more on this in a minute. Essentially, "a decidual cast occurs when your body expels the inside lining of your uterus, which is known as the endometrium, all at once instead of gradually shedding the tissue," says Landry. The medical term is "membranous dysmenorrhea," and it can happen during your period.

Why does it happen?

This mysterious symptom is kind of a head-scratcher even for doctors, and it's not exactly known what causes it. "We don't know the exact incidence, but decidual cast is not common," says Landry. "It seems to happen more frequently in people who use hormonal birth control."

Langdon notes it can also occur after you've discontinued using hormonal birth control, adding that it can also happen during an ectopic pregnancy. For a quick refresher, an ectopic pregnancy occurs when a fertilized egg implants and grows outside the main cavity of the uterus. If left untreated, an ectopic pregnancy is potentially life-threatening, so it isn't a bad idea to seek medical attention if you think your decidual cast might be related to an ectopic pregnancy.

What are some signs of a decidual cast?

Decidual cast can also have related symptoms, such as heavy bleeding, cramping, or even light spotting in the days leading up to the passage of the tissue. You might experience pain due to the cervix dilating to pass the tissue. Thankfully, these symptoms typically subside quickly after the cast exits the vagina. Of course, if you do have any concerns, checking in with your doctor is always a solid move.

It can also happen to a person more than once, though there isn't concrete data on how many people experience decidual cast multiple times.

What should you do if you think you've experienced this?

"The decidual cast tissue can sometimes resemble tissue that is expelled during a miscarriage. Therefore, if you could be pregnant, take an at-home pregnancy test," says Landry.

"If you save the cast, you can have it analyzed by a pathologist," adds Langdon. "To do that, you must put it in a plastic bag and bring it to your doctor. Refrigerate it if there will be a delay in bringing it to the lab or office."

The TL;DR on this mystifying menstrual phenomenon, per Landry: "A decidual cast isn't harmful, but because there are many other reasons for heavy, painful periods, you should let your clinician know if you experience a decidual cast or have any questions or concerns about your period."