The short answer
Mixed dentition is the stage when a child has both primary (baby) teeth and permanent teeth in the mouth. It usually begins around age 6 and continues through much of the school-age transition, ending after the remaining primary teeth have been replaced.
The mouth can look uneven during this phase because teeth do not loosen, fall out, and erupt in perfectly matched pairs. A newly erupted permanent tooth may sit beside smaller primary teeth, one side may be ahead of the other, and first permanent molars may appear at the back without replacing any baby tooth.
This can be part of ordinary development, but appearance alone cannot show whether every tooth is present, positioned well, or following a healthy path. Routine dental assessment matters.
What changes during mixed dentition?
The transition is not one event. It is a sequence:
- Front primary teeth loosen and shed.
- Permanent incisors erupt into those spaces.
- First permanent molars emerge behind the primary molars.
- Later, primary canines and molars are replaced by permanent canines and premolars.
- Second permanent molars generally erupt near the later part of the transition.
These steps overlap. There may be quiet periods between visible changes, and the upper and lower arches follow different patterns. A separate Oral Compass eruption-timeline guide is being prepared for broad age-range orientation; until it is published, a dentist can interpret an individual child’s sequence.
Why the mouth can look uneven
During mixed dentition, families may notice:
- gaps where a baby tooth was lost;
- permanent front teeth that look large beside remaining baby teeth;
- teeth at different heights because they erupted at different times;
- one side changing before the matching side;
- a permanent tooth appearing behind or to the side of a primary tooth; or
- new permanent molars at the very back.
None of these observations, by itself, diagnoses crowding or an eruption problem. Some are temporary parts of the transition; others deserve monitoring. The child’s dentist looks at the full pattern, available space, tooth position, bite, health of the primary teeth, and development over time.
The “six-year molars” do not replace baby teeth
First permanent molars are sometimes called six-year molars because they commonly erupt around ages 6 to 7. They appear behind the last primary molars. Because no baby tooth falls out to make room for them, families may mistake them for additional baby teeth.
They are permanent teeth and need careful brushing as soon as they break through the gums. Their chewing surfaces can be difficult for a child to reach while they are only partly erupted. A dental professional can assess cleaning, cavity risk, and whether preventive measures are appropriate.
Does uneven mean something is wrong?
Not necessarily. Mixed dentition is uneven by nature: two sets of teeth with different shapes are present while the jaws are still growing and teeth are emerging at different times.
But “common” is not the same as “always normal.” A photo or age cannot determine whether a tooth is blocked, missing, extra, erupting along an unusual path, or affected by early primary-tooth loss. The AAPD recommends evaluating the developing dentition as part of comprehensive oral health care so findings can be interpreted in context.
What a dentist monitors
Depending on the child’s stage and findings, the dental team may monitor:
- whether primary teeth are loosening and shedding in a coherent sequence;
- whether permanent teeth are emerging and can be kept clean;
- space and crowding patterns;
- the child’s bite and jaw development;
- teeth that appear off-path, retained, missing, or extra; and
- effects of decay, injury, habits, or early tooth loss.
Monitoring does not automatically mean treatment is needed. It means comparing today’s findings with the child’s history and development. Imaging is selected only when the dentist judges that it is indicated; there is no universal mixed-dentition X-ray schedule.
When to ask for an earlier assessment
Contact the child’s dental office rather than waiting for a routine visit if there is:
- persistent pain, swelling, drainage, or difficulty eating;
- a permanent tooth erupting while the baby tooth remains firmly in place and the pattern concerns you;
- a baby tooth lost early because of decay or injury;
- a tooth that seems unable to emerge or is difficult to clean;
- a pronounced difference between the two sides that persists; or
- an injury or sudden change in tooth position.
These signs do not establish a diagnosis or predict treatment. They give the dental team a reason to assess the individual pattern. If a baby tooth is loose, avoid forcing it; see what to do about a loose baby tooth. If a baby tooth was lost early, this guide explains why timing and location matter.
Caring for a mixed set of teeth
Keep the routine simple:
- brush twice daily with fluoride toothpaste in the amount recommended for the child’s age;
- help or supervise until the child can clean every surface reliably;
- pay extra attention to partly erupted molars and crowded or overlapping areas;
- clean between teeth where they touch; and
- keep regular visits based on the child’s needs and the dentist’s advice.
Primary teeth still matter while they remain. They support function and help hold space during the transition. Why baby teeth matter explains that role in more detail.
The takeaway
Mixed dentition is the normal bridge between the primary and permanent sets. It often looks untidy because the exchange happens tooth by tooth, not all at once. Gaps, different tooth heights, and a temporary mix of shapes can occur—but a chart or appearance cannot diagnose the child’s development. Regular care and a dentist’s assessment turn an uneven-looking phase into a monitored, understandable transition.
Sources
- Management of the Developing Dentition and Occlusion in Pediatric Dentistry — American Academy of Pediatric Dentistry
- Eruption Charts — American Dental Association MouthHealthy
- Baby Teeth — American Dental Association MouthHealthy
- Periodicity of Examination, Preventive Dental Services, Anticipatory Guidance/Counseling, and Oral Treatment — American Academy of Pediatric Dentistry