Discovery of Identical Twins Following Cancer Diagnosis
Two sisters from Cardiff, Caitlin and Grace Leggett, who had always believed they were fraternal twins, discovered they were actually identical twins after Caitlin was diagnosed with leukaemia at age 23.
Growing up, Caitlin and Grace were very close, sharing many hobbies and attending the University of Bristol together, where they lived in the same student house. They had always understood themselves to be dizygotic twins, meaning they originated from two separate eggs fertilised by different sperm.
However, when Caitlin was diagnosed with leukaemia in April, and Grace prepared to donate stem cells, medical tests revealed they were monozygotic twins—identical twins formed when a single fertilised egg splits into two.
"It can happen to anyone, really," Caitlin said regarding her diagnosis. "Obviously I didn't expect anything to happen to me."
The sisters expressed that discovering they were identical was "a bit cooler" than being fraternal twins, although this revelation complicated Caitlin's treatment because Grace was unable to donate stem cells. They are now urging more people to register as stem cell donors.
Caitlin's Diagnosis and Initial Treatment Plan
Before her diagnosis, Caitlin had planned to join the Army as an intelligence officer and had scheduled her medical assessment for April.
In March, she developed a persistent rash that did not improve with over-the-counter creams. Blood tests ordered by her general practitioner led to an urgent hospital referral the next day.
She was admitted to the Teenage Cancer Trust unit at the University Hospital of Wales in Cardiff, where she was diagnosed with acute myeloid leukaemia (AML), a fast-progressing cancer affecting white blood cells.
The treatment plan involved chemotherapy followed by a stem cell transplant to replace the cancerous cells with healthy ones.
Stem Cell Donor Search and Identification of Twin Type
Both Caitlin and Grace had registered as stem cell donors with DKMS several years prior. Family members were tested to find a suitable match.
Initially, Grace was considered a likely donor due to their sibling relationship, which typically increases the chance of compatibility.
"I had to go to one of the hospitals in Cardiff and they did all the checks - my weight, they took around 15 little vials of blood to test for all different diseases and everything, checked heart rate and lungs," Grace explained. "Everything was fine, so they gave me a little pamphlet with all the information."
However, doctors later discovered that every genetic marker tested was identical between the sisters, leading to the conclusion that they were identical twins.
"I always wished I was an identical twin - I liked being a twin, but being an identical twin is a bit cooler," Grace said.
Caitlin and Grace were in separate amniotic sacs during pregnancy, which led their parents to believe they were fraternal twins. However, identical twins can also develop in separate sacs.
Impact on Treatment and Subsequent Developments
While the revelation was unexpected, it complicated Caitlin's treatment. Doctors advised that Grace could not donate stem cells because a successful transplant requires the recipient's body to recognise donor cells as foreign to attack remaining cancer cells.
Since Caitlin and Grace share nearly identical DNA, the transplant would not have the intended effect.
The planned transplant for August was cancelled as doctors searched for an alternative donor. During this period, Caitlin discovered the leukaemia had spread to her skin and enrolled in a clinical trial in Manchester to address this.
The trial initially succeeded, but the cancer returned shortly before a second transplant scheduled for December, necessitating additional full-body radiotherapy.
"The radiation was probably the worst," Caitlin said. "I didn't feel anything from chemo initially, but the first day of radiation, oh my God, I thought I was dying. It was crazy. I had, like, all the side effects on one day, but then by the second day, I got used to it, so I felt alright."
In December, Caitlin underwent a successful stem cell transplant from an anonymous donor.
Encouragement to Register as Stem Cell Donors
Both sisters are now advocating for more people to register as stem cell donors to help others in need.
"It takes maybe six hours maximum (to donate), and then for someone else, it changes their life," Grace said. "So I don't see why you wouldn't do it."
