What Is a Decidual Cast? Everything You Need to Know About This Menstrual Condition

You know sexual health education is a problem when even nurses are stunned by their bodies during menstruation. In a viral TikTok video, Iowa-based nurse Madi Swegle described her experience with a rare, but extremely painful decidual cast.

“Something extremely scary and honesty a little traumatic happened to me last night,” she said in her post. “This is important. I’ve never heard of this before in my life [and]I’m a nurse…I wish I had, which is why I’m posting this video.”

After a few hours of starting what felt like a normal period, Swegle says her cramps got “suddenly got very, very intense.” After moving to the toilet with a heating pad and bucket for her nausea, Swegle describes the intense cramping as “the worst pain I’ve experienced in my entire life.” Eventually, Swegle passed a clump of tissue from the vagina.

The tissue was the size of her palm and triangular — the shape of a uterus. After sending a picture to her OBGYN, Swegle discovered she had passed a decidual cast, or the lining of her uterus.

What Is a Decidual Cast?

A decidual cast describes a large piece of tissue that passes through the cervix and vaginal canal — the tissue is the lining of the uterus that is shed during menstruation. As Shieva Ghofrany MD, OBGYN, and co-founder of TRIBE CALLED V explains, “[A decidual cast] is essentially when the uterine lining sloughs off in one entire piece.” The term refers to the “cast” shape of the uterine cavity, and the “decidua” refers to the mucus lining of the uterus that forms every month during the menstrual cycle, continues Ghofrany.

Shedding this lining is a completely normal part of menstruation, per Cleveland Clinic. During the menstrual cycle, the uterus prepares for pregnancy by thickening its walls with an influx of hormones, mainly estrogen and progesterone, says Stanford Medicine. The lining of the uterus is called an endometrium. (You may be reminded of endometriosis, which describes a condition where the endometrium grows outside the uterus).

When pregnancy does not occur, the body needs to pass the endometrium during your period. Typically, the lining is shed over the course of menstruation (usually 5-7 days) in smaller fragments, along with blood and mucus.

However, sometimes — though rarely — the lining is shed is one intact piece. In these cases, the lining appears as one piece of triangular-shaped tissue, resembling the uterus.

Does Decidual Cast Mean Miscarriage?

A decidual cast is not the same thing as a miscarriage. A miscarriage is defined as a spontaneous loss of a pregnancy before the 20th week, per Mayo Clinic.

However, Dr. Ghofrany notes that decidual casts can occur with ectopic pregnancy — or when a fertilized egg grows outside the main cavity of the uterus and is therefore not a viable pregnancy.

One study published in the Internet Journal of Gynecology and Obstetrics notes the strong association between ectopic pregnancy and decidual casts, while also stating there are cases of non-pregnant people (those assigned female at birth) experiencing this condition. In this case, it’s most common to see a decidual cast in non-pregnant people who are using human menopausal gonadotrophin (HMG), human chorionic gonadotrophin (HCG), and progestogens — all three of which are hormonal medications.

If you do pass tissue and are unsure if you’ve experienced a decidual cast or miscarriage, Dr. Ghofrany recommends calling your doctor and taking a pregnancy test. Your doctor may recommend a pelvic ultrasound to ensure that all looks normal, Dr. Ghofrany says.

Why Did I Pass a Decidual Cast?

It’s unclear why exactly a decidual cast happens, but it is associated with contraceptives (particularly progesterone contraceptives, says Dr. Ghofrany), as they affect the hormones in your body.

A handful of studies — including this 2022 case report in PubMed — note the correlation between the “initiation or cessation of hormonal treatments” (like birth control, for example) and decidual casts.

However, keep in mind that decidual casts are very rare. If you’re concerned about experiencing this, talk to your doctor before stopping your birth control. If you’re feeling nervous about contraceptive care and your risk for decidual casts, Dr. Ghofrany says, “there are no specific patients who need to avoid contraceptives for this particular reason.”

However, if you do experience a decidual cast — or more than one — you should consult your doctor.