A baby boy in Brazil has been born with a 12-centimeter-long (5 inches) "true" human tail, which are extremely rare with only 40 cases reported in scientific literature. The new case is described in the Journal of Pediatric Case Reports.

The boy, who has been kept anonymous in the report, was born prematurely but without any other complications. Upon examination, he was found to be jaundiced and had a "rounded fibroelastic appendage of approximately 4 centimeters in its largest diameter, supported by a 12 centimeters fibrous cord in the left paravertebral lumbosacral region", the doctors write in the report. In layman's terms, a tail.

The newborn was examined for other possible systemic alterations, none of which were revealed during an ultrasound.

Tails on humans are classified into "true" tails or "pseudo-tails".

"Pseudo-tails are protuberances basically composed of adipose or cartilaginous tissue and the presence of bone elements," the team report. "True human tails are very rare, with approximately 40 cases reported in the literature."

"True" or "real" tails refer to embryonic tails that remain until birth. In usual development, embryos form a small tail at around four weeks, which are then absorbed by white blood cells at six to 12 weeks.

In incredibly rare cases such as this, the tail is not broken down by white blood cells and remains when the fetus comes to term. Though the team removed the tail with no complications, "true" tails often contain muscle tissue, and can (in some cases) be twitched or even curled by their human.

Before the tail was removed, the team investigated other potential conditions that could be present in the boy.

"Due to the common ectodermal origin between the skin and the central nervous system, it is essential that the pediatrician or pediatric surgeon investigate the presence of hidden spinal dysraphism in patients with suspected skin lesions, as they may be the only visible abnormality and early diagnosis can prevent evolution to severe neurological changes," the team wrote.

Spina bifida is often associated with having a true tail, but this infant was found to have no other conditions. The team did not present a cause for the tail, though they note that the mother was treated with first-generation cephalosporin for a urinary tract infection, and continued to smoke 10 cigarettes a day during pregnancy.

Following the removal of his tail, the team reported no further problems for the boy, and he was perfectly healthy.