What Is a Decidual Cast?

Medically Reviewed by Melinda Ratini, MS, DO on August 08, 2023
3 min read

A decidual cast is a large, intact piece of tissue that you pass through your vagina in one solid piece. It happens when the thick mucus lining of the uterus, called the decidua, sheds in the near exact shape of your uterine cavity, creating a triangular “cast.”

The medical term that doctors use for this is membranous dysmenorrhea. It can happen when you’re having your period.

Doctors aren’t exactly sure what causes it. But they do believe it can be a sign of an ectopic pregnancy (a pregnancy that forms outside your uterus) or related to hormonal contraceptives with progesterone.

Passing a decidual cast can be very painful. In the days before you pass it, you may have symptoms such as:

But once you pass the cast, your symptoms may go away almost immediately. Once out, the cast looks pink, fleshy, and solid. It may have a shiny appearance because it’s made up of tissue, mucus, and blood clumped together.

So far, there have only been a handful of cases of people passing a decidual cast. Their ages have ranged from 9 to 41 years old. Because of the limited information and research, doctors don’t know what the exact cause is, but they believe it could be associated with:

Ectopic pregnancy. This is a type of pregnancy that happens when the embryo implants itself outside of your uterus. It’s most common in one of your fallopian tubes, but can also happen in the ovaries or the abdominal cavity. If the embryo ruptures, it can cause internal bleeding and can be life-threatening. You can’t carry an ectopic pregnancy to term. You’ll need medical attention right away.

Hormonal contraceptives. Decidual casts may also have a link to the use of hormonal birth control that contains progesterone. These contraceptives can be taken by mouth, injected, or implanted in your body. One case study noted that in all reports of decidual casts, the women were using or had just stopped using a hormonal contraceptive.

Before they confirm a decidual cast, your doctor also has to rule out other conditions that may cause similar symptoms.

This can include:

  • Benign or cancerous tumors
  • Polyps
  • Pregnancy
  • Miscarriage
  • Ectopic pregnancy

Due to the lack of information on decidual casts in current medical research and OB/GYN textbooks, it may also be an underdiagnosed condition.

If you have a decidual cast, you’ll usually pass it in one piece. But your doctor will do a transvaginal ultrasound of your uterine cavity to make sure all of the tissue has come out.

Once you pass it, your symptoms will stop almost immediately. A decidual cast isn’t a signal of a serious condition. There’s no record of long-term negative side effects, and there’s also very little risk that you’ll have another one.

If you notice any pain or cramping, pass lumps of tissue from your vagina, or have an irregular pattern of bleeding, let your doctor know about it as soon as possible.

If you have any side effects from taking hormonal birth control, tell your doctor before you switch or stop taking it. They may be able to prescribe something that works better for you.