It is a rare occurrence. Medically they are called the 'vanishing twin', a freak delivery in which one the twins appears to be painted on a transluscent leaf-like sheath.
On Friday last week, a young woman in Kolkata delivered 'vanishing twin', a medical wonder that has not been reported in India ever.
Twenty-four-year-old Amita Dutta delivered the twins -- one healthy boy and another 'flat' lifeless form that looked more like a fossilised baby.
"This is really amazing... we had read about it in our medical books, but never heard anything like this being reported in the country," said Dr Swagata Mukherjee, who assisted the normal delivery at the Howrah Jain Hospital.
The medicos had documented a twin after three months of Amita's pregnancy through pre-natal ultrasonography.
"However, at nine months, we could hear just one of the twin hearts beat. That put us on alert," he said.
After the delivery, Mukherjee and Jayati Bardhan of the North Bank Diagnostic Centre scanned medical literature from across the world to find out that the occurrence, variously described as 'vanishing twin', 'twin embolisation syndrome' or foetus papiricus (leaf-like foetus), had not been reported from this part of the world.
Normally two sperms fertilising two ova give rise to two zygotes, which simultaneously grow in one sack inside the mother's womb to be born as twins called dizygotic monochorionic twins (two zygotes, one sack).
"But this incident was also rare in that the twins were dizygotic dichorionic (two zygotes developing in two sacks)," Mukherjee said.
Unaware of the medical importance of the case, Amita and husband Pradyut Dutta, residents of Salkia in Howrah, are happy that at least one of the twins was a normal healthy baby.
While the 'vanishing twin' has been preserved at the hospital for medical reference, the doctors are preparing a paper to report the rare case in a medical journal.