Decidual Cast: What a 2021 Case Report Revealed, and What It Didn't

Six lines into the case report, the detail that stops you isn't the tissue. It's the pregnancy test.

In March 2021, three clinicians at East Sussex Healthcare NHS Trust in England published an account of a patient who passed a decidual cast: the entire lining of her uterus, shed in one piece, still holding the shape of the cavity it came from. The paper ran in BMJ Case Reports and logged two firsts. It was the first cast reported to have become infected while still inside the body, and the first found to contain old products of conception.

Her urine and serum beta-hCG tests were both negative. Chorionic villi turned up on histology anyway.

That gap is why a two-page case report deserves more attention than its length suggests. Decidual casts occupy a strange corner of medicine: uncommon enough that plenty of doctors never see one, common enough that TikTok has a hashtag. A scoping review published in the International Journal of Gynecology & Obstetrics, with its literature search closing in February 2026, found 113 publications on the subject going back to 1724. Eighty-six were single case reports or small series. That is the entire evidence base.

What a decidual cast actually is

Start with the ordinary cycle. After ovulation, the corpus luteum, the temporary gland left behind in the ovary, produces progesterone. Progesterone drives decidualisation: the endometrium thickens, its stromal cells swell, and the tissue rearranges itself to receive a blastocyst and help build a placenta. If no pregnancy begins, the corpus luteum regresses, progesterone falls, and that prepared lining breaks down and leaves the body over several days. This is a period.

Sometimes the breakdown doesn't happen. The lining stays intact, detaches as a sheet, and is pushed through a cervix that has to open far wider than it would for blood and clot. What comes out is firm, reddish or pink, and often triangular or pear-shaped, because it was moulded by the uterine cavity. Clinicians at the Cleveland Clinic describe casts ranging from the size of a walnut to a small lime. Patients' accounts tend to be larger.

The pain is the part people remember. It behaves more like labour than like a bad period, and it stops once the tissue is out. Madi Swegle, a labour and delivery nurse in the United States, described an hour of contractions, nausea and chills before passing palm-sized tissue in the shape of her uterus. She had never heard of the condition, and neither had several colleagues she messaged.

The clinical name is membranous dysmenorrhoea, and it is not new. Eighteenth-century physicians described it; journals have catalogued the casts ever since.

The East Sussex case, and why two firsts matter

The patient had used depot medroxyprogesterone acetate (DMPA), the injectable contraceptive sold as Depo-Provera, for five years, then stopped. DMPA is given every 13 weeks and is up to 99.8% effective with perfect use. It is also one of the most widely used contraceptives in the world: researchers at EPI-PHARE, the French group that studies medicine use, estimate 74 million women receive it each year.

Progestin withdrawal is a recognised trigger for cast formation in the case literature. The BMJ authors noted that nobody had previously reported it after stopping DMPA specifically. Their patient arrived with signs of infection. Imaging showed the cast sitting partly inside the uterus and partly in the posterior fornix of the vagina, and her infection markers settled only once the tissue came away. The team treated her with intravenous antibiotics, analgesia and antiemetics, and waited for spontaneous passage rather than operating. They concluded the cast itself was the likely source of infection, not pelvic inflammatory disease, since the CT showed no hydrosalpinx and no adnexal inflammation. The full report is openly available from BMJ Case Reports.

The pregnancy question was harder. With negative urine and serum tests, the working assumption was no pregnancy. Histology overturned it. The authors offered two possible chronologies: she conceived after stopping DMPA and miscarried without passing the pregnancy tissue, with the cast forming around it; or she conceived months earlier, while still injecting, and the cast formed later. Without serial scans and beta-hCG values from those months, neither can be confirmed.

They did not overclaim. Their own summary of the field is that there is "significant room for further research" into why some DMPA users form casts and most don't.

Photo by prashant hiremath on Unsplash

Here's the catch: a cast can hide a pregnancy

The textbook rule is clean enough. If someone has a positive pregnancy test and passes tissue with no chorionic villi in it, the pregnancy is somewhere else, usually a fallopian tube. Steven Goldstein, a professor of obstetrics and gynaecology at NYU Grossman School of Medicine, put the logic plainly to TODAY: if the tissue holds no pregnancy, where is the pregnancy? A tubal ectopic can rupture a week or two after a cast passes.

That rule depends on the test being positive. Here it wasn't, twice, on urine and on serum. Rare, and the authors say so. The lesson isn't that beta-hCG is unreliable. It's that a cast plus bleeding plus abdominal pain earns a proper workup even when the numbers look reassuring.

There is a longer paper trail of exactly that failure mode. In 1982, clinicians at King Edward Memorial Hospital in Perth described the pseudogestational sac: a decidual cast with blood trapped in its centre that mimics a gestational sac on ultrasound. Of six cases recorded over three years, three were flagged as possible ectopics because of suspicious extrauterine echoes. In the other three, the ultrasonologist reported an intrauterine pregnancy, an error that changed how those patients were managed.

The trap still closes. In 2024, a case report in Cureus described a woman whose ectopic pregnancy was misread on imaging as a heterotopic one, a twin pregnancy with one sac in the uterus and one in the tube. She had a laparoscopic salpingectomy for the ectopic and a dilatation and evacuation for the "failed" intrauterine pregnancy. Histopathology on the uterine contents found decidua and no trophoblastic tissue. There had never been a second pregnancy.

None of this is new. A 1958 review of eight years of ectopic diagnoses by a single gynaecologist found five false positives out of 26, and noted that four patients passed decidual casts, two of whom did not have an ectopic pregnancy at all.

Grading the hype: the vaccine surge claim

In April 2022, a paper in the Gazette of Medical Sciences claimed an "unprecedented rise" in decidual cast shedding during 2021 and floated COVID-19 vaccination as a cause. It is the claim that still circulates. It deserves a proper accounting.

The numbers came from an online survey run by MyCycleStory: 292 self-reported casts out of 6,049 respondents, or 4.83%, with a mean age of 36.1 and 85% of respondents identifying as non-Hispanic white. Roughly one in six reported prior use of an IUD or hormonal contraception. There was no control group, no clinical confirmation of most reports, and no denominator, which means no way to say how many unvaccinated women would have reported the same thing. The scoping review that mapped this literature classifies the paper as "one social media survey combined with a Google trend analysis" and flags "significant selection bias".

Compare that with properly controlled menstrual data. An NIH-funded US cohort of 3,959 people using the Natural Cycles app found vaccination associated with a change in cycle length of less than one day and no change in bleeding duration. The global follow-up, covering 19,622 people and nine vaccines, found increases of 0.71 days after a first dose and 0.56 days after a second; people who received both doses in a single cycle saw 3.91 days. Changes resolved within a cycle or two, as the NIH's own summary sets out. Neither study looked at decidual casts at all.

The verdict: the vaccine hypothesis is a hypothesis built on a biased survey and published outside the mainstream gynaecological literature. It has not been replicated, and the controlled data show menstrual effects far smaller and shorter-lived than the paper's framing implies.

What three centuries of case reports add up to

The scoping review is the most complete picture anyone has assembled. Of 113 included publications, 121 cases were described individually, plus another 42 from a 1904 series. Patients ranged from 9 to 53 years old. The studies came from 23 countries, with the United States contributing 47, the UK 18 and India 10, and no publications at all from Africa, which tells you something about whose cases get written up. The review is open access in IJGO.

Its conclusions are modest, and the authors say why: the literature is almost entirely case reports, publication bias is strongly suspected, and overall certainty of evidence is rated "very low".

Three findings are still worth carrying. First, the pattern has shifted: historically most cases were spontaneous and recurrent, while recent decades are dominated by sporadic cases tied to hormonal treatment. Second, hormones cut both ways. The same drug classes implicated as triggers have been used since the 1950s to suppress recurrent membranous dysmenorrhoea, with sequential oestrogen-progestogen regimens reported to help where continuous combined therapy made symptoms worse. Third, hormonally triggered cases rarely recur, and most women can keep using their chosen method.

Management, in the review's own summary, is analgesia and patient education, with dilatation and curettage reserved for selected cases. Concomitant infection is rare, which is exactly what makes the East Sussex case worth publishing.

Photo by KOBU Agency on Unsplash

If it happens to you

The distance between what clinicians know and what patients need is where most of the distress sits. Swegle's account is instructive precisely because she is an obstetrics nurse and still had no idea what she was looking at.

What helps, in order:

• Take a pregnancy test. A negative result makes an ectopic pregnancy unlikely; a positive one changes the whole picture and needs same-day assessment.
• Keep the tissue if you can. Photograph it, seal it in a bag, refrigerate it if there's a delay, and take it to your appointment. Histology is what separates a cast from a miscarriage or a gestational sac.
• Report what you've taken. Recent starting, stopping or missed doses of any hormonal contraceptive, whether pill, implant, injection or hormonal IUD, is the most useful history you can give.
• Go now, not later, if you have fever, pain that doesn't settle after passage, bleeding that soaks a pad an hour, or faintness.
• Expect a conversation rather than a procedure. For a confirmed cast with no pregnancy and no infection, analgesia and reassurance are the treatment.

One caveat, from the review: the condition "might be underreported, as it is rather unknown among clinicians." You may be the first person to say the words out loud to your doctor. Patient accounts like Swegle's have done more to change that than most journal articles.

What the researchers want next

Two things are missing, and both are structural rather than scientific. The first is counting. Nobody knows how often decidual casts happen, because there is no registry, no reporting pathway, and no strong reason for a patient to bring tissue to a clinic once her pain has stopped. The second is mechanism. Why the endometrium fails to dissolve in one woman and behaves normally in the millions of others on the same injection remains unanswered.

The review's authors want systematic case collection and larger cohorts to pin down recurrence risk and the best management. Fukaura, Ward and Datta want the same thing from the other direction: a clearer picture of who forms a cast, and whether expediting passage, medically or surgically, beats waiting when the tissue is infected.

Neither question gets answered until clinicians recognise what is in front of them. Correct diagnosis, the review's authors write, is what prevents unnecessary intervention and gives women accurate information. That is a low bar. It is also the whole game.