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A decidual cast refers to the shedding of the entire uterine lining (the decidua) all at once. During a normal menstrual cycle, the uterine lining is shed over a period of three to five days and appears as a menstrual flow. When a decidual cast occurs, the uterine lining comes out whole. It resembles the shape of your uterus, which looks like an upside down triangle or a lightbulb. The event is also known as membranous dysmenorrhea.

It's not known how often decidual casts occur, but they are thought to be rare. They are not dangerous in and of themselves but can be shocking nonetheless, and it’s understandable to have concerns if you experience it. Here's everything you need to know about a decidual cast—including symptoms, causes, diagnosis, and management.

What Does a Decidual Cast Look Like?

When you pass a decidual cast, you are shedding the uterine lining that builds up each month before your period.

Normally, this thick lining is shed once a month, over a span of several days, when you are experiencing your period. When you have your period, the uterine lining comes out looking like flowing blood, though it may have some clots or chunky bits.

But when the uterine lining comes out whole like in the case of a decidual cast, it may look like a fleshy mass or blob. You’ll see blood as well as grayish-brown or whitish-gray tissue. There may also be cyst-like areas in the cast.

The decidual cast will be in the shape of your uterus. The general shape of the uterus is a “V” or an upside down triangle. Some people describe the shape as a lightbulb or an upside-down pear. In adults, a uterus is about 3 inches long and 2 inches wide. That's about the size a decidual cast would be too.

Symptoms of a Decidual Cast

Passing the decidua can be very painful. In fact, the most common physical symptom of a decidual cast is extreme abdominal pain. The pain is caused by uterine contractions that cause cramps, as well as the uterine lining passing through a closed (non-dilated) cervix.

Some people have reported that the pain of a decidual cast is so significant that over-the-counter (OTC) pain relief doesn’t help.

Causes of a Decidual Cast

Because there are so few documented cases of decidual casts, experts aren’t entirely sure what causes them. There are several theories as to what causes them, though.

Some experts have theorized that significant changes in hormones—specifically, a sharp increase in both estrogen and progesterone—might cause the spontaneous expelling of the uterine lining.

Abnormalities in the production of prostaglandin, a hormone-like substance, may contribute to a decidual cast. It's believed that endometrial (uterine) infections may trigger a decidual cast, too.

Risk Factors

Because decidual casts are so rare, there is limited research on them. As such, it’s not clear what the risk factors are for the condition. While more research is needed to say for sure, it's believed that certain medications, specifically those that contain the hormone progesterone, may be a risk factor for decidual casts.

According to one report, virtually all people who are known to have had a decidual cast were on hormonal birth control or hormonal therapies of some kind. Most of the recorded cases specifically involve people who had taken progesterone-containing contraceptives. Some of the people had recently stopped hormonal therapy. The majority of people were between the ages of 20-40.

How Is a Decidual Cast Diagnosed?

If you have intense cramping that isn’t relieved by OTC medication and is making it difficult to perform everyday tasks, you should visit a healthcare provider. If you’ve passed what looks like a decidual cast, you should call your healthcare provider. It can be helpful to preserve the decidua and/or take pictures of it.

The main way that decidual casts are diagnosed are by a histopathological exam, which is when the tissue is examined under a microscope so that it can be correctly identified. In addition, the following tests may be performed:

• Ultrasounds

• Pregnancy tests

• Pelvic exams

• Blood tests

These tests will look at the general health of your reproductive system and test for infections, pregnancy, uterine masses, and any conditions you might be experiencing that could be causing or contributing to your symptoms.

How to Manage a Decidual Cast

You will likely not know that you are passing your decidua until after it’s happened. But if you are experiencing intense menstrual cramps, you should consider trying to relieve the pain with anything you might have at home. This could include OTC pain relievers or heating pads. A hot water bottle on your stomach area can help too. If at-home pain relief measures aren't helping, consider getting in touch with your healthcare provider or going to urgent care for pain management help.

If you have passed a decidual cast, you should call your healthcare provider. They may ask you to save the tissue or go in for an exam. If you are on birth control, you should continue taking it unless your healthcare provider says otherwise. Some providers may change your medication dose or type if they believe it may have contributed to a decidual cast.

Complications of a Decidual Cast

Although decidual casts are alarming, they are not considered dangerous. They don’t have any known long-term complications. It’s also unlikely that you will have another decidual cast.

Even so, you should always connect with your healthcare provider if you pass a decidual cast. You should have the condition properly diagnosed, and you may need to discuss a care plan, which may involve changing the type of birth control you use.

A Quick Review

A decidual cast is when your uterine lining—called the decidua—sheds all at once. When this happens, you will pass a uterus-shaped mass through your vagina. While the sight of the decidua can be frightening because it is bloody, fleshy, and may look like an organ, decidual casts are not harmful. However, they can be very painful and you might experience severe cramping because of it.

It's unknown what exactly causes a decidual cast, but it's believed that changes in hormones are involved. Taking hormonal birth control or hormonal therapies might increase your risk of the event. Overall, decidual casts are uncommon. But if you do experience one, report your experience to your healthcare provider so you can discuss any potential next steps.

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