Twin sisters from Cardiff who believed for decades they were fraternal twins discovered they were actually identical after being diagnosed with leukemia at the age of 23.
For most of their lives, 23-year-old Caitlin Leggett and Grace Leggett believed they were fraternal twins (born from two separate eggs fertilized by two different sperm).
They were always very close as twin sisters. They grew up together in Cardiff and shared many of the same interests, including competing in track and field together for years.
Both went on to study at Bristol University, Caitlin studying psychology and Grace studying neuroscience, coincidentally completing a master’s degree in stem cells and regeneration, and the sisters shared student accommodation and remained best friends.
After graduation, Caitlin wanted to train as an intelligence officer and planned to join the Army. She had booked the necessary medical evaluation for April 2025, but in March of that year she developed a rash that did not go away despite using creams she bought at the pharmacy.
Believing she might have the virus, she consulted her GP and ordered a blood test to investigate, which was carried out on April 3, 2025.
That night, at around 12:30am on April 4, Caitlin received an urgent call from her out-of-hours GP who told her to go straight to A&E. At the hospital, she was told to come back the next morning for an appointment, but then she received a call to report to the Llewellyn Teenage Cancer Trust department at the University Hospital of Wales in Cardiff instead, and she soon realized it wasn’t good news.
In fact, at that appointment she was diagnosed with acute myeloid leukemia, an aggressive cancer of the white blood cells.
The NHS said symptoms of AML include a pale complexion, feeling tired or weak, shortness of breath, unusual and frequent bruising or bleeding, and effortless weight loss, but Caitlin said she didn’t feel any of these and the unusual rash was the only sign that something was wrong.
She was told that treatment would include not only chemotherapy but also a stem cell transplant to replace cancer cells destroyed by chemotherapy with healthy stem cells.
Caitlin and her sister Grace had registered as donors on the DKMS stem cell registry “many years ago,” and following Caitlin’s diagnosis, her entire family underwent tests to see if anyone in the family was eligible.
Initially, Grace was said to be a perfect fit. Because she is a sibling, there is a good chance that her stem cells will be accepted into Caitlin’s body and help her recovery.
“I had to go to hospital in Cardiff and all the tests were done there. They took my weight, they took about 15 little vials of blood, they tested me for various diseases and everything else, they checked my heart rate and my lungs,” Grace explained.
“Everything was fine and they gave me a little pamphlet with all the information.
“And all that was left to do was wait.”
A short time later, Caitlin received a call from a consultant who explained that after looking at Grace’s results and a “triple test,” they found that “all the markers we looked for were exactly the same, and if they weren’t the same, you’d expect different things,” Caitlin explained, adding that doctors had concluded that they were in fact identical twins.
Caitlin and Grace said their parents believed they were fraternal twins after being told they were in separate amniotic sacs in the womb. However, identical twins can and often do develop in separate amniotic sacs.
“I always wished I was an identical twin. I liked being a twin, but being an identical twin is a little cooler,” Grace said.
“It’s really weird to think that I’m basically Caitlin!”
Although the twins were overjoyed to learn that they were closer than they had imagined, the discovery also meant a setback for Caitlin’s treatment.
Grace was unable to donate her stem cells to Caitlin because doctors advised that for the transplant to be successful, the body would recognize the stem cells as foreign and the donor cells and patient cells would need to attack each other. The hope is that the donor cells will “win” the battle and kill the leukemia. Because Caitlin and Grace have nearly identical DNA, Caitlin said, “the whole concept of a transplant wouldn’t work.”
Caitlin has already undergone chemotherapy and thankfully “didn’t experience any side effects.” Her stem cell transplant was planned for August 2025, with Grace expected to be the donor.
But when that proved impossible, doctors were forced to find an alternative donor from the registry. Initially they found a donor, but had to withdraw the donor, during which time Caitlin learned that the leukemia was in her skin.
Due to the discovery, a transplant planned for August was canceled and Caitlin took part in a clinical trial in Manchester to remove the leukemia in her skin. The transplant date was postponed in stages, one week at a time, so Caitlin could not receive additional chemotherapy during that time.
Thankfully, the trial went well and she was given another transplant date with a different donor in December 2025. Luckily, she had three partial matches, all of whom were eligible donors.
However, one week before this second transplant date, the cancer returned to her skin. Doctors decided to proceed with the transplant anyway, but Caitlin required additional whole-body radiation therapy.
“The radiation was probably the worst,” Caitlin said, adding that the first day was especially bad.
“At first I didn’t feel anything with the chemotherapy, but the first day of radiation I thought, oh my god, I’m going to die. It was really crazy.
“One day I had some side effects, but by the second day I got used to it and felt okay.”
In December 2025, Caitlin finally received a successful stem cell transplant from an anonymous donor. She did not suffer from many side effects. Although he experienced infections and some stomach issues, he said, “It wasn’t as bad as most people.”
After a successful transplant, Caitlin is currently undergoing clinical trials for new drugs, but there is still a long way to go when it comes to treatment. Her bone marrow will be monitored once a month for three months, and if those tests are positive, her bone marrow tests will be reduced to once every three months, followed by regular lab and blood tests.
Grace and Caitlin both encourage others to register as stem cell donors. According to DKMS, more than 2,000 people in the UK require a blood stem cell transplant each year, but only 7% of the UK’s total eligible population are registered as potential donors.
“Just like donating blood, if at some point in the future I need blood, I want it to be available. So it’s kind of like that. If I need stem cells in the future, for something like a transplant, I want to have blood available if I can,” Grace said.
“(Donating) may take up to six hours, but then it’s life-changing for someone else. So I don’t see why you wouldn’t do it.”
“The truth is, it can happen to anyone,” Caitlin added.
“Of course I didn’t expect anything to happen to me…
“It could literally happen to anyone.”
For more information about DKMS and registering as a stem cell donor, please visit: www.dkms.org.uk.