A decidual cast is when the body sheds the decidua — the thickened uterine tissue — all at once. When a person passes the tissue, they may experience pain similar to period cramps.

During a typical menstrual period, the body sheds a portion of the tissue that lines the uterus in small pieces and blood. A decidual cast means that instead of shedding the tissue as a period, the body discards the entire layer in one piece.

A decidual cast is usually pear or triangle-shaped. Although it is often harmless, it can be a sign of pregnancy loss or an ectopic pregnancy.

Read on to learn what a decidual cast is and what to do about it.

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This feature mentions pregnancy loss, stillbirth, or both. Please read at your own discretion.

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The decidua is the thickened line of the endometrial tissue, which lines the uterus.

Each month, the body sheds the decidua gradually as blood and tiny bits of tissue. When the body sheds the tissue all at once, this is a decidual cast.

A person may also have cramping, which can sometimes be intense, as the body passes the tissue.

Case reports of decidual casts describe a large chunk of tissue, much larger than a typical blood clot a person might pass with their period. It is often about the size of the palm of the hand and is triangle or pear-shaped.

While typical clots are thin and tissue-like, a decidual cast is a thick, solid mass. They are usually bloody and come out in one solid or mostly solid piece.

A decidual cast itself is not dangerous. However, some people experience heavy bleeding or anemia because of the large and sudden loss of blood.

Decidual casts are more common in people taking medications containing progesterone, especially in hormonal birth control. They can also occur as a complication of pregnancy loss or an ectopic pregnancy.

If a person’s period is late or delayed, or they have recently had a positive pregnancy test, the cast could be a sign of these issues. However, even if a person’s period is on time, a decidual cast requires medical attention, since an ectopic pregnancy can be dangerous without treatment.

An early ectopic pregnancy might cause no symptoms other than a decidual cast.

A decidual cast can sometimes occur for no apparent reason. Many case reports feature decidual casts in young females or very young adults, but no large body of research suggests there is a correlation with age.

Some other potential causes include:

  • Progesterone: Medications containing progesterone may increase the risk of a decidual cast. Many case reports of this diagnosis include a person who was taking progesterone, usually in the form of hormonal birth control.
  • Pregnancy loss: Sometimes, pregnancy loss, especially one that does not expel from the body, can cause a decidual cast. A 2021 case report details a case in which a person developed an infection in a decidual cast after a delayed loss of pregnancy.
  • Ectopic pregnancy: A decidual cast may occur along with an ectopic pregnancy.

A decidual cast does not refer to pregnancy loss. However, it can occur alongside the latter.

Some differences between a decidual cast and pregnancy loss include:

  • A person does not have to be pregnant to have a decidual cast. Pregnancy loss only occurs during pregnancy.
  • A pregnancy loss can pass slowly, as blood, and does not always include a large mass. A decidual cast, by definition, always involves a large mass.
  • A decidual cast is not an embryo or fetus. It is old uterine tissue.

Some signs that a decidual cast could be occurring along with pregnancy loss include:

  • A person recently had a positive pregnancy test.
  • A person has missed their period or recently had sex without a barrier method such as a condom.
  • A person has had a change in their periods or thinks they might be pregnant.

A decidual cast is not necessarily dangerous. However, it can occur alongside emergencies such as ectopic pregnancy and pregnancy loss. Therefore, it is important to seek medical care even if a person feels fine.

A person needs to contact a doctor if:

  • They experience heavy vaginal bleeding, whether or not it is time for their period.
  • They suddenly pass a very large tissue through their vagina.
  • They feel weak or faint while bleeding vaginally.

A decidual cast can cause concern, especially if a person does not know what it is. However, it is not usually dangerous or a sign of a serious underlying condition.

However, it could mean a person is pregnant. It may also occasionally occur with excessive bleeding.

A person needs to seek prompt care for large tissue that passes from the vagina or for any heavy vaginal bleeding.