Transitional dentition is the phase when a child has both baby teeth and permanent teeth in their mouth at the same time. It typically spans from around age 6, when the first permanent molars come in, to age 12 or 13, when the last baby tooth falls out. Dentists also call this the “mixed dentition” stage, and it’s one of the most active periods of change in a child’s mouth.
How Baby Teeth Make Way for Permanent Ones
The process starts well before a baby tooth gets wiggly. Inside the jaw, the roots of baby teeth gradually dissolve as the permanent teeth developing beneath them push upward. By age 6, all 20 baby tooth crowns are still visible in the mouth, but root resorption is already happening below the gumline. Once enough root material has dissolved, the baby tooth loosens and falls out, and the permanent replacement moves into position.
This isn’t random. The body follows a fairly predictable sequence, though every child varies by several months in either direction.
When Each Permanent Tooth Arrives
According to the American Dental Association’s development chart, permanent teeth emerge in a general order:
- Ages 6–7: Lower central incisors (the bottom front teeth) and the first permanent molars, which appear behind the last baby teeth rather than replacing any
- Ages 7–8: Upper central incisors
- Ages 7–9: Lower and upper lateral incisors (the teeth flanking the front ones)
- Ages 9–10: Lower canines
- Ages 10–12: Upper and lower premolars, which replace the baby molars
- Ages 11–12: Upper canines and second premolars
Girls tend to be slightly ahead of boys in this timeline. If one side of your child’s mouth is more than six months ahead of the other, that asymmetry is worth mentioning at a dental visit.
The “Ugly Duckling” Stage
Sometime around ages 8 to 10, many kids develop a gap between their upper front teeth and their incisors may flare outward or look slightly crooked. This appearance has a name: the “ugly duckling” stage (formally called the Broadbent phenomenon). The hallmark features include a midline gap between the two front teeth, upper incisors that tip sideways, and front teeth that angle forward.
It looks alarming, but it’s almost always temporary. The permanent canines, still buried in the jaw at this point, put pressure on the roots of the incisors as they migrate downward. Once those canines fully erupt (usually by age 12), the front teeth are pushed into better alignment and the gap closes on its own. One study found that 83% of children who had a midline gap at age 9 had no gap by age 16. Starting orthodontic treatment specifically for this gap during the mixed dentition phase is often unnecessary and can actually complicate things.
Leeway Space and Why It Matters
Baby molars are wider than the premolars that replace them. The size difference leaves a small amount of extra room in each side of the jaw, called “leeway space.” In the upper jaw, this averages about 0.9 mm per side. In the lower jaw, it’s larger: roughly 2 mm per side.
That may sound tiny, but those millimeters are valuable. The body uses leeway space to allow the permanent teeth to settle into proper alignment. Orthodontists sometimes plan treatment around preserving or strategically using this space. If a baby molar is lost too early (from decay or injury), the teeth behind it can drift forward and consume the leeway space before the premolar is ready to come in, leading to crowding.
Ectopic Eruption: When Teeth Stray Off Course
One of the more common complications during transitional dentition is ectopic eruption, where a permanent tooth drifts off its normal path and gets stuck against the tooth in front of it. This happens most often with the first permanent molars, which can wedge under the back edge of the baby molar instead of emerging straight up into the chewing surface.
Signs that suggest ectopic eruption include a permanent molar that’s more than six months late compared to the same tooth on the opposite side, or an X-ray showing the molar tilted forward. If it locks against the baby molar, it can erode the baby tooth’s root, potentially exposing the nerve or causing the baby tooth to be lost years ahead of schedule. That premature loss creates a domino effect of spacing problems.
The good news is that many ectopic molars self-correct between ages 7 and 8 as jaw growth provides more room. When they don’t, a dentist can intervene with relatively simple techniques to redirect the tooth before the situation becomes more complicated.
Why Age 7 Is the Key Orthodontic Checkpoint
The American Association of Orthodontists recommends every child have their first orthodontic evaluation by age 7. At that point, the mix of baby and permanent teeth gives an orthodontist a clear picture of how the jaw is growing and whether the permanent teeth have enough room. Problems like crossbites, severe crowding, or jaw discrepancies are easier to address when growth is still happening.
This evaluation doesn’t mean treatment starts at 7. Most kids who are screened at that age won’t need braces until more permanent teeth have come in. But catching certain issues early, particularly skeletal problems where the upper and lower jaws aren’t growing in proportion, can reduce the complexity of treatment later.
Keeping Mixed Dentition Healthy
The transitional dentition phase creates a uniquely challenging landscape for brushing. Teeth sit at different heights as they emerge, gaps from recently lost baby teeth collect food, and the first permanent molars appear so far back that kids often miss them entirely. Those first molars are especially vulnerable because they erupt with thinner enamel than they’ll eventually have, and they arrive in a location that’s hard to reach with a child-sized toothbrush.
The most practical step is making sure your child brushes all the way to the very last tooth in each row, angling the bristles toward the gumline of those back molars. Loose baby teeth also need attention: kids tend to avoid brushing near a wiggly tooth because it feels strange, but the gum tissue around it is prone to inflammation if plaque builds up. Fluoride toothpaste remains important throughout this stage, as newly erupted permanent teeth absorb fluoride readily and benefit most from it during their first years in the mouth.