Children develop a set of 20 primary teeth, commonly called baby teeth or milk teeth. These 20 teeth are split evenly between the upper and lower jaws: eight incisors, four canines, and eight molars. The first usually appear around six months of age, and the full set is typically in place by the time a child turns three. That number stays stable until around age six, when permanent teeth begin pushing through and the count temporarily climbs during a transitional stage called the mixed dentition.
When Baby Teeth Come In
The lower central incisors, the two small front teeth on the bottom, are almost always the first to break through the gums. The upper central incisors follow shortly after, then the upper lateral incisors, lower lateral incisors, first molars, canines, and finally the second molars. A cross-sectional study of eruption timing found that this sequence held for both boys and girls, though the exact age at which each tooth appeared varied between the sexes. The entire process from first tooth to last took about 22 months on average regardless of sex.1PubMed Central. Eruption Chronology in Children: A Cross-sectional Study
Most parents notice the lower front teeth arriving between five and ten months, but there is a wide window of normal. Some babies cut their first tooth at four months; others do not see one until after their first birthday. Pediatric dentists generally do not worry about late eruption unless a child reaches 18 months with no teeth at all, at which point an X-ray can check whether the teeth are developing normally beneath the gums.
How Baby Teeth Differ from Permanent Teeth
Baby teeth are not just miniature versions of adult teeth. They are structurally distinct in ways that affect how they respond to cavities, fillings, and everyday wear. The enamel layer on a primary tooth is roughly half as thick as on a permanent tooth. One study measured the average enamel thickness of baby teeth at about 1.14 mm compared with about 2.58 mm for permanent teeth, and found that permanent enamel contained a higher percentage of calcium and phosphorus.2PubMed. Microstructure and mineral composition of dental enamel of permanent and deciduous teeth That thinner, less mineralized shell is one reason cavities can progress faster in baby teeth than in adult teeth.
Beneath the enamel, the dentin layer also differs. Primary teeth have a much higher density of tiny tubules running through the dentin, roughly two to three times the density found in permanent teeth.3Microscopy and Microanalysis. Tubule Density and Diameter in Coronal Dentin from Primary and Permanent Human Teeth Additionally, the crystalline structure and grain size of both enamel and dentin are smaller in baby teeth than in permanent ones, which has practical implications for how dental filling materials bond to them.4Advanced Materials Research. The Crystal Structure and Mechanical Characteristics of Enamel and Dentin of Primary and Permanent Teeth In short, baby teeth are softer and more porous. They do a fine job for the few years they need to last, but they are not built for the long haul.
Why Baby Teeth Fall Out
Around age six, children start losing their primary teeth. The process looks simple from the outside: a tooth gets wiggly and eventually pops out. Underneath the gum, though, something more orchestrated is happening. The developing permanent tooth sends chemical signals that trigger specialized cells to gradually dissolve the root of the baby tooth above it. This root resorption process uses many of the same molecular pathways the body uses to remodel bone.5Journal of Oral Science. Physiologic root resorption in primary teeth: molecular and histological events As the root is eaten away bit by bit, the baby tooth loses its anchor and loosens until the child can pull or push it out with minimal effort.
The lower front teeth are typically the first to go, followed by the upper front teeth, mirroring the order in which they arrived. The molars are last to shed, usually around age 10 to 12. Most children lose all 20 baby teeth by age 12 or 13, replaced by 28 permanent teeth (32 if wisdom teeth eventually develop). The period between roughly age 6 and 12, when a child has a mix of baby and permanent teeth in the mouth, is called the mixed dentition phase.
The Mixed Dentition Phase
During the mixed dentition years, a child’s mouth can look like a construction zone. Baby teeth, gaps, and larger permanent teeth all coexist. At its peak, a child might have anywhere from 24 to 28 teeth in the mouth depending on how many baby teeth have fallen out and how many permanent ones have arrived. Dentists pay close attention to this phase because the way baby teeth shed and permanent teeth come in sets the stage for how the adult bite lines up.
A longitudinal study tracking children from the primary dentition through the transition into mixed dentition found that roughly three-quarters developed a normal bite relationship between their first permanent molars, while about a quarter developed some degree of misalignment.6PubMed. Longitudinal occlusal changes during the primary dentition and during the passage from primary dentition to mixed dentition among a group of Turkish children The way the baby molars fit together before they are lost influences how the permanent molars settle into position. This is one reason pediatric dentists sometimes recommend early orthodontic evaluation around age seven, even though braces may not be needed until years later.
Why Keeping Baby Teeth Healthy Matters
A common misconception is that baby teeth do not matter much since they are going to fall out anyway. In reality, primary teeth serve as natural space holders for the permanent teeth forming beneath them. When a baby tooth is lost too early, whether from decay, injury, or infection, the neighboring teeth can drift into the gap and block the permanent tooth from coming in straight.7PubMed Central. Orthodontic Space Management in Pediatric Dentistry: A Clinical Review The British Dental Journal has highlighted how premature loss of primary teeth can have lasting effects on the developing bite, often requiring orthodontic correction later.8British Dental Journal. Space maintainers in the primary and mixed dentition – a clinical guide
The consequences go beyond crooked teeth. Research has linked premature loss of baby teeth to a meaningful drop in oral health-related quality of life for preschool children, with one study finding that children who lost posterior baby teeth prematurely had roughly two and a half times the negative impact on daily function compared with those who had not.9PubMed Central. Premature Primary Tooth Loss and Oral Health-Related Quality of Life in Preschool Children Interproximal cavities between baby molars also cause measurable loss of space in the dental arch, and the more surfaces affected, the greater the space lost.10PubMed Central. Interproximal caries and premature tooth loss in primary dentition as risk factors for loss of space in the posterior sector When a dentist does extract a baby tooth early, they often place a small appliance called a space maintainer to hold the gap open until the permanent tooth is ready to come through.
When the Count Is Off
Not every child ends up with exactly 20 baby teeth or 32 permanent ones. Some children are born missing one or more teeth entirely, a condition called hypodontia. This is more common in the permanent dentition than in the primary set, and it appears slightly more often in girls than in boys.11PubMed Central. Congenitally missing teeth (hypodontia): A review of the literature concerning the etiology, prevalence, risk factors, patterns and treatment A retrospective study of patients with congenitally missing teeth found that the vast majority of absent teeth were permanent rather than primary, with only 12 missing baby teeth identified out of 503 total missing teeth in the sample.12PubMed. Clinical features of hypodontia and associated dental anomalies: a retrospective study The most commonly absent permanent teeth tend to be the second premolars and the upper lateral incisors. When a baby tooth has no permanent successor waiting beneath it, the primary tooth sometimes stays in place well into adulthood, lasting decades beyond its expected lifespan.
On the other end of the spectrum, some children develop extra teeth. The most well-known supernumerary tooth is the mesiodens, an additional tooth that appears between the two upper central incisors. Its prevalence varies across populations but generally falls somewhere between about 0.1% and 2%.13PubMed Central. Diagnosis and management of supernumerary (mesiodens): a review of the literature Extra teeth can crowd the arch, block normal teeth from erupting, and sometimes require surgical removal. They are found more often in boys and are usually detected on routine dental X-rays before they cause visible problems.
Babies Born with Teeth
In rare cases, a baby is already born with one or more teeth, called natal teeth, or develops them within the first month of life, called neonatal teeth. Natal teeth are about three times as common as neonatal teeth.14PubMed Central. Natal and neonatal teeth: an overview of the literature These teeth are almost always lower central incisors and are typically part of the normal set of 20, not extras. A case-control study of 52 children with natal or neonatal teeth confirmed that all of them were mandibular central incisors.15Scientific Reports. Natal and neonatal teeth in newborns and infants: a case-control study
The main concern with natal teeth is that they can be very mobile, raising the risk of the tooth detaching and being inhaled. In that study, about 31% of children with natal teeth required extraction because the teeth were too loose to leave safely in the mouth.15Scientific Reports. Natal and neonatal teeth in newborns and infants: a case-control study Other potential problems include difficulty breastfeeding and ulceration of the baby’s tongue from the sharp edge of the tooth. When the tooth is stable and not causing feeding problems, most clinicians leave it alone.16PubMed Central. Management of an infant having natal teeth
How Nutrition and Health Affect Tooth Timing
The 20-tooth, six-months-to-three-years timeline is an average, and several factors can shift it. Nutrition is one of the biggest. A study of Filipino children aged 10 to 13 found that those who were stunted or underweight had significantly fewer permanent teeth erupted than their well-nourished peers at the same age, suggesting that malnutrition delays the transition from primary to permanent dentition.17PubMed Central. Relationship between Malnutrition and the Number of Permanent Teeth in Filipino 10- to 13-Year-Olds Chronic illness, hormonal differences, and genetic conditions can also speed up or slow down tooth development. Children with certain syndromes may be missing teeth altogether or may have them erupt in unusual patterns.
Premature birth is another factor. Babies born very early sometimes show delayed eruption of their primary teeth by several months. On the other hand, some systemic conditions and endocrine disorders can cause teeth to appear earlier than expected. The takeaway for parents is that the textbook timeline is a guide, not a rulebook. A child who is a few months ahead or behind the average is nearly always within normal range.
Using Teeth to Estimate a Child’s Age
Because tooth development follows such a predictable biological clock, dental professionals can estimate a child’s age quite accurately by looking at which teeth have erupted and how far along the roots are in development. This is useful in forensic settings, in refugee and asylum cases where birth records are unavailable, and in pediatric medicine when a child’s exact age is uncertain.
One widely used tool is the London Atlas, which matches a panoramic dental X-ray to reference images showing typical tooth development at each age. A study applying this method to Malaysian children found the mean difference between the estimated dental age and the child’s actual age was only about 0.6 years, with no significant difference between the sexes.18PubMed Central. Applicability of London atlas of tooth development and eruption for dental age estimation in children of the Malaysian population using maxillofacial imaging Other approaches focus on the maturation of specific teeth, particularly the third molars, to determine whether an individual has reached the age of legal majority. One analysis found that correct classification of minors versus adults was achieved in about 82% of cases, though the accuracy was lower for females, and researchers caution that overestimation of age remains a concern for people close to 18.19PubMed Central. Dental Age Assessment Based on Developmental Stages and Maturity Index of Third Molars for Medico-Legal Purposes
Why Mammals Get Only Two Sets
If you have ever wondered why humans get just baby teeth and then permanent teeth, rather than growing new teeth continuously the way sharks or crocodiles do, the answer lies deep in mammalian evolution. The two-set system, known as diphyodonty, is a hallmark of mammals. It appears to have evolved alongside precise occlusion, the tight interlocking fit between upper and lower teeth that allows mammals to chew food efficiently. Continuously replacing teeth would make it hard to maintain that precise fit, so mammals traded the ability to keep generating new teeth for a set that, once in place, fits together reliably for a lifetime of chewing.
Paleontological evidence pushes the origin of this two-set pattern back to at least the Late Triassic period, more than 200 million years ago. A study of the fossilized animal Brasilodon found that its dental replacement pattern shared key features with modern mammals, including deciduous teeth with larger, more complex crowns being replaced by smaller, simpler permanent teeth.20PubMed Central. Diphyodont tooth replacement of Brasilodon—A Late Triassic eucynodont that challenges the time of origin of mammals The deciduous teeth in Brasilodon even showed less mineralized enamel compared with the permanent replacements, a pattern still seen in modern children. Humans, in other words, are working with a dental blueprint that is hundreds of millions of years old.
Traditions Around Lost Baby Teeth
The experience of losing teeth is so universal in childhood that cultures around the world have developed rituals to mark it. The Tooth Fairy is probably the most familiar in Western countries, and a systematic review of cultural traditions around exfoliated teeth found that 56% of the 37 scholarly works identified on the topic mention the Tooth Fairy specifically.21PubMed Central. Cultural Diversity of Traditions for the Disposal of Exfoliated Teeth: Implications for Researchers But the review also documented a much wider range of practices globally, organized into seven broad themes. In many cultures, children throw the lost tooth onto the roof or into the ground, sometimes accompanied by a wish for a strong replacement. In parts of the Middle East, the tooth is thrown toward the sun. Some traditions involve burying the tooth or giving it to an animal figure rather than a fairy.
These rituals serve a practical purpose beyond entertainment: they help children cope with the strange experience of losing a part of their body. The promise of a coin under the pillow or a wish granted by tossing the tooth into the sky turns a potentially unsettling event into something to look forward to. Researchers studying these traditions have noted that the sparse scholarly attention paid to non-Western tooth disposal customs means we probably know only a fraction of the rituals that exist worldwide.