Thanks to millions of years of evolution, the human body has become well versed in its developmental processes. Most of the time, it gives us the correct number of organs and structures that we need to function. But sometimes, hiccups in this process mean people end up with extras.
Author
Adam Taylor
Professor and Director of the Clinical Anatomy Learning Centre, Lancaster University
Accessory teeth, fingers, toes and nipples are actually surprisingly common.
Up to are born with extra fingers or toes, while around are born with additional teeth. Both of these phenomena are – particularly extra digits, which are in males.
of people have an extra nipple. These can appear anywhere along the line that runs from the armpit to the groin – known as the . As with extra teeth and digits, supernumerary nipples are , but it’s unclear why. They also tend to appear on the left side of the body.
But these aren’t the only places that extra body parts can appear. Some people are born with extra bones, organs or genitalia they might not even be aware they have.
Ribs
Supernumerary ribs are typically seen in two places: the neck or the lumbar region (lower back). An estimated 1% of the population has an extra rib – though when it comes to an extra neck rib, it could actually be closer to . The extra rib can occur on one or both sides of the spine.
An extra rib usually causes no problems and is only during an X-ray or imaging for another condition – although it can cause harm if it compresses neighbouring nerves and blood vessels.
If this happens in the neck, it may cause in the arms. In the lumbar region, additional ribs can .
Abdominal organs
An extra spleen is surprisingly common. and of people have an accessory spleen.
They’re typically found – but in some circumstances can be dragged down into the pelvis. Accessory spleens can present clinical challenges if they need to be surgically removed in the treatment of , where red blood cells are destroyed faster than they can be made.
The liver is the abdomen’s largest organ, comprising four different parts (lobes). Less than are reported to be born with an accessory lobe of the liver – though this number could be higher, as having an accessory lobe usually causes zero symptoms. Most people who have an additional liver lobe only discover it incidentally – either during or for a different condition.
But there are cases of the accessory lobe causing a torsion . This cuts off the blood supply and requires .
Supernumerary kidneys can also occur – though this is , with less than 100 reported cases on record. Again, this may be because they tend not to cause any health problems. There’s also the rarest-of-the-rare: a recent baby was , making four in total.
What is more common, however, is having a duplex kidney – where there’s an additional tube coming from the kidney that connects the bladder. Up to has one.
Genitalia
There are multiple genital anomalies that can happen. This is due to the fact that the testes and ovaries develop on either side of the body, making it possible for supporting structures to fuse incompletely during normal development, causing supernumerary genitalia to form.
Double uteri happens in about , a very rare occurrence. Each uterus can have its own cervix and vagina (or may share a vagina). Each uterus is usually capable of carrying a healthy – though a double uterus has been associated with of miscarriage, preterm delivery, breech delivery and stillbirth.
can also happen in rare cases. Often, these are found or , where they can .
Men can also be born with additional genitalia. Diphallia refers to being born with two penises – a phenomenon which occurs in . Each , with varying size, shape and functionality.
Recently, it was even reported a child had been born with (triphalia) – although only one had all the necessary tubes to urinate.
Supernumerary testicles (polyorchidism) may also be seen, with three being the most common anomaly – with the extra one usually being side. Throughout history, there have only been seven reported cases of a person being . Additional testicles do not cause issues unless they haven’t descended into the scrotum, which is associated with an .
are an incredibly rare duplication, with only .
Extra limbs
Additional limbs (polymelia) are one of the rarest supernumerary structures seen in humans. This happens in the upper limbs almost than the lower limbs.
In the upper body, these accessory limbs tend to , while in the lower body additional legs tend to . can also occur.
In even rarer circumstances, an additional limb can grown from the – known as . But rarest of all is “cephalomelia” – where an infant is born with limbs . In these cases, surgery will be required to remove the extra limb.
Supernumerary structures of all kinds typically don’t cause any harm to our health. In many cases, they’re just another thing that makes each human unique.
Adam Taylor does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
