London(CNN) A pair of Pakistani conjoined twins are finally living independent lives after a 100-strong team of British medical experts spent 50 hours performing complex surgery to separate them.
Safa and Marwa Ullah came as a bit of a surprise when they were born in January 2017 as their mother had not known she was expecting twins -- let alone craniopagus twins, who are joined at the head.
Around two in five sets of craniopagus twins are stillborn or die during labor, while a third more do not survive the first 24 hours. Fortunately in the sisters' case, a wealthy benefactor offered to cover the costs of their long journey from home in Charsadda, Pakistan, for the surgery at London's Great Ormond Street Hospital (GOSH).
According to its website, GOSH "is one of only a few hospitals in the world to have the infrastructure, facilities and team of experts" to separate conjoined twins. It has carried out the procedure more times than any other hospital worldwide.
Born by cesarean section, the girls emerged with their skulls and blood vessels fused together. Since arriving in Britain, they have undergone three operations between October of last year and when they were finally separated on February 11.
Such a procedure is extraordinarily rare; according to GOSH, the chances of craniopagus twins undergoing surgery is around one in 10 million. Around 5% of conjoined twins are craniopagus cases.
After consultation with their doctors in Pakistan, the London hospital welcomed them to the Bumblebee Ward where various experts -- from craniofacial, neurology and psychology specialists, to nurses, radiologists and physiotherapists -- spent four months treating them.
Consultant neurosurgeon Noor ul Owase Jeelani and craniofacial surgeon Professor David Dunaway led a team of 100 staff members.
Now, five months after the last operation, the hospital has told the twins' tale. A short animation video released by GOSH explains the procedure, while separate footage sees the medical team recount their efforts.
Jeelani explained that conjoined twins are "very, very unusual," but even more so those joined at the head.
The secret of the "very complex procedures," according to Jeelani, was breaking it down to smaller and "much more manageable steps."
The experts used virtual reality to create an exact replica of the girls' anatomy in order to visualize their skulls and the positioning of their brains and blood vessels.
"For the first two to three procedures we focused on separating out the brains and blood vessels," Jeelani said in the GOSH video.
Once this was completed, a piece of plastic was used to keep the two structures apart. "Internally we had two separate kids," the neurosurgeon added.
The focus was then external, as the team used the girls' own bone at the top of their skulls and "tissue expanders" to stretch their skin over their heads.
Though the girls are said to be doing well, the surgery was not plain sailing. During one of the operations they started to bleed after clots formed in Safa's neck and she began to transfer blood to Marwa, reported the UK's Press Association (PA).
When Marwa's heart rate fell, doctors feared they would lose her -- so they decided to give her a key vein she shared with her sister. This resulted in Safa suffering a stroke less than 12 hours later.
As a staff nurse on Bumblebee ward, Lydia Lowe got to know the children well. "Safa has always been a bit boisterous; she's always been the first to count or speak," she recounted in the GOSH video.
Lowe described Marwa as "more cheeky", adding: "As soon as Safa goes to sleep Marwa comes out to shine."
Having been discharged on July 1, the girls are said to be doing "well" despite the many challenges ahead. Their mother Zainab Bibi, 34, is currently residing with them in London where they are undergoing daily physiotherapy. Their father died of a heart attack before they were born, but their uncle and grandfather are on hand to help.
Bibi, who had seven children prior to the twins, said in a statement: "We are indebted to the hospital and to the staff and we would like to thank them for everything they have done. We are extremely excited about the future."
Jeelani said he was optimistic that the twins would be walking by their third birthday.
"From our personal point of view, it has been great to get to know the girls and their family. Their faith and determination have been so important in getting them through the challenges they have faced. We are incredibly proud of them," Jeelani and Dunaway added in a statement.