A Brazilian baby boy was born with a "true" human tail and had the appendage surgically removed, according to a case report.
The unnamed child, who was born prematurely via an uncomplicated vaginal delivery 35 weeks into his mother's pregnancy, came out of the womb jaundiced and with a tail, a case study in the Journal of Pediatric Surgery Case Reports revealed.
The appendage was around 12 centimeters (4.7 inches) long and had a ball-shaped mass at the end that measured 4 centimeters (1.6 inches) in diameter, according to the report published in March.
All fetuses develop an embryonic tail while in the womb around the fourth week of gestation, but it is usually reabsorbed back into the body by the eighth week, becoming the tailbone or the coccyx, the study said.
The Brazilian newborn, however, still had his embryonic tail — dubbed a "true" human tail — intact upon being born. It was composed of adipose and connective tissue, blood vessels as well as muscle and nerve fibers.
"True human tails are very rare, with approximately 40 cases reported in the literature," the report said.
"Pseudo-tails," on the other hand, are "protuberances basically composed of adipose or cartilaginous tissue and the presence of bone elements," according to the study.
The baby was later scanned with an ultrasound to rule out any neurological involvement as the nervous system and skin derive from a common embryonical origin, Fox News reported.
He was then taken to an operating room in Albert Sabin Children's Hospital in the Brazilian city of Fortaleza after testing negative for any nervous system involvement, and his tail was successfully removed without complications, according to the outlet.
The study did not disclose why the family opted to remove the tail, and no details regarding the surgical procedure were revealed.
The child's mother was described as someone who was previously healthy and did not drink alcohol or use illicit drugs. She did, however, smoke 10 cigarettes per day during her pregnancy and had a urinary tract infection during her first trimester, which was treated with antibiotics.
Humans' ape ancestors are believed to have shed their tails around 25 million years ago, a report by the New York Post said.