An underbite is a type of misaligned bite where the lower teeth and jaw sit in front of the upper teeth when the mouth is closed. Roughly 6% of the global population has one, making it the least common of the three main bite classifications. Underbites range from barely noticeable to severe, and the distinction matters because mild cases may only need braces while significant ones can require surgery.
What Happens in Your Jaw
In a normal bite, the upper front teeth overlap the lower front teeth slightly. With an underbite, that relationship is reversed. The lower jaw extends forward so the bottom teeth close in front of the top teeth, sometimes by a little, sometimes dramatically.
There are two distinct types. A skeletal underbite means the jawbones themselves are the problem: either the lower jaw grew too large, the upper jaw didn’t grow enough, or both. A dental underbite means the jaw bones are roughly the right size and position, but the teeth are angled in a way that creates the misalignment. Knowing which type you have determines the treatment path. Dental underbites are generally simpler to correct. Skeletal ones often need more involved intervention, especially in adults.
What Causes an Underbite
Genetics is the primary driver. The shape and size of your jaw and teeth are largely inherited, and most people with bite misalignment can trace it to family traits. If one or both parents had an underbite, their children are significantly more likely to develop one.
Childhood habits play a role too. Prolonged thumb-sucking, extended pacifier use, and long-term bottle-feeding all place repeated pressure on teeth while they’re still forming, which can push them out of alignment. Tongue-thrusting, where a child presses their tongue against the back of the front teeth while swallowing or speaking, and chronic nail-biting can have similar effects.
Less commonly, facial injuries and tumors in the mouth or jaw can cause an underbite. Even after surgical repair from an injury, the jaw bones don’t always grow back into perfect alignment.
How Common Underbites Are
A large systematic review of global data found that about 5.93% of people with permanent teeth have an underbite, with a wide range depending on geography. Prevalence runs as low as 0.7% in some Israeli populations and as high as 19.9% in parts of China. In children with a mix of baby and adult teeth, the rate is slightly lower at about 4%, and the highest figures in that age group appear in Asian populations at roughly 5.8%. By comparison, the most common bite pattern (where teeth align normally or close to it) accounts for about 75% of people worldwide.
Effects on Speech and Daily Life
An underbite changes the way your tongue, teeth, and lips interact, which can affect how you pronounce certain sounds. Research in The Angle Orthodontist found that people with underbites have particular difficulty with “s,” “z,” “l,” and “r” sounds. This happens because the lower jaw’s forward position forces the tongue to sit lower than usual, both at rest and during swallowing, which interferes with the precise tongue placement those sounds require.
Chewing can also become less efficient. When the upper and lower teeth don’t meet properly, breaking down food takes more effort, and certain foods become harder to bite into. Over time, the uneven contact between teeth accelerates enamel wear on the surfaces that do meet, which can lead to chipping, sensitivity, and increased cavity risk. The strain on the jaw joint from compensating for the misalignment can contribute to jaw pain, clicking, headaches, and the broader set of symptoms associated with temporomandibular joint disorders.
How Underbites Are Diagnosed
An orthodontist typically evaluates an underbite using a combination of physical examination, dental molds, and X-rays. The most informative imaging is a lateral cephalometric radiograph, a side-view X-ray of the skull that allows precise measurement of the angles between the upper jaw, lower jaw, and the base of the skull.
Three key angles tell the story. The SNA angle measures the upper jaw’s position relative to the skull base (average: 81 degrees). The SNB angle does the same for the lower jaw (average: 78 degrees). The difference between them, called the ANB angle, indicates the skeletal relationship. A normal ANB angle is about 2 degrees. When it drops below 2 degrees, it signals an underbite pattern. These measurements help the orthodontist determine whether the underbite is skeletal, dental, or a combination, and how severe it is.
The American Association of Orthodontists recommends that all children be screened by an orthodontist by age 7. At that age, enough permanent teeth have come in to identify developing problems, but the jaw is still growing and responsive to early treatment.
Treatment in Children
Because a child’s jawbones are still forming, early intervention can redirect growth and potentially avoid surgery later. One of the most common tools is a palatal expander, a device that fits against the roof of the mouth and gradually widens the upper jaw. Orthodontists often recommend starting one around age 7 or 8, when the two halves of the upper jaw haven’t yet fused.
A rapid palatal expander attaches to the back upper teeth and has a small screw in the center that a parent turns daily with a special key. It widens the jaw at a rate of about half a millimeter per day. For children who need only minor widening, a removable version that looks like a chrome retainer may be sufficient. Other devices used in early treatment include reverse-pull headgear, which pulls the upper jaw forward, and chin cups that gently restrain lower jaw growth.
The goal of early treatment isn’t always to finish the job. Sometimes it’s about making enough of a correction during the growth window that any future treatment is simpler and less invasive.
Treatment in Teens and Adults
For dental underbites where the jaw structure is reasonably aligned but the teeth aren’t, braces or clear aligners can often produce good results in 12 to 24 months without surgery. The orthodontist repositions the teeth so the upper arch closes over the lower arch properly.
Skeletal underbites in adults are a different challenge. Once the jaw has finished growing, typically by the late teenage years, bone can no longer be reshaped with appliances alone. Orthognathic (jaw) surgery becomes the primary option. The process usually involves 12 to 18 months of braces before surgery to align the teeth within each arch so they’ll fit together correctly once the jaw is repositioned.
During the surgery itself, the surgeon works mostly through incisions inside the mouth, making precise cuts in the jawbone to reposition it, then securing everything with surgical screws, plates, or wires. A splint is placed over the teeth for stability. Recovery takes several weeks, and a second phase of orthodontic treatment follows to fine-tune the bite. The full process from start to finish, including pre-surgical braces, surgery, and post-surgical adjustments, typically spans two to three years.
Skeletal vs. Dental: Why It Matters
The single most important factor in how an underbite is treated is whether it originates in the bone or the teeth. A dental underbite, where the jaw is the right size but teeth are tilted or crowded into the wrong position, can often be corrected with orthodontics alone at any age. A skeletal underbite, where the lower jaw is genuinely too large or the upper jaw too small, needs growth modification in children or surgery in adults to achieve a stable correction. Trying to fix a skeletal problem with braces alone tends to produce results that don’t last, because the underlying jaw discrepancy remains.
If you’re unsure which type you or your child has, the cephalometric X-ray measurements described above are what distinguish them. That’s the starting point for any treatment plan, and it’s why an orthodontic evaluation is more informative than trying to assess severity in a mirror.