Baby teeth and adult teeth are not just different sizes of the same thing. They differ in number, structure, eruption pattern, and, importantly, in what they need from a parent’s care routine. Understanding those differences makes the whole job of looking after a child’s mouth considerably easier. Let’s explore the practical differences every UK parent should know, from how many teeth your child will eventually have, to why a chipped baby tooth still matters even though it’s destined to fall out. As part of our children’s dental care at our practice, we have this conversation with parents almost daily. NHS dental treatment is free for children under 18 in England.
The numbers, briefly
A complete set of baby teeth, also called the primary or deciduous dentition, has 20 teeth: ten on the top, ten on the bottom. They typically all arrive between six months and 30 months of age.
A complete adult set has 32 teeth, including four wisdom teeth that come through (or don’t) between the ages of 17 and 25. Most of the adult teeth start replacing baby teeth from around age six and the process is largely complete by age 13, with the second molars usually erupting at around 12.
The transitional period, when both baby and adult teeth are present in the mouth, is called the mixed dentition phase. It runs roughly from age six to twelve and is when most orthodontic concerns are first spotted.
Why baby teeth matter more than parents think
The single most common misconception we encounter is the idea that baby teeth don’t really matter because they’re going to fall out anyway. It’s an understandable assumption. It’s also wrong.
Baby teeth do four important jobs:
- They allow proper chewing, which affects nutrition and speech development
- They hold space for the adult teeth coming through underneath
- They guide the eruption of those adult teeth into the right position
- They support the development of clear speech, particularly the front incisors
If a baby tooth is lost early, through decay or trauma, the teeth either side can drift into the gap. That blocks the adult tooth’s path and can cause crowding or misalignment that requires orthodontic treatment later. The baby tooth was, in effect, holding the parking space.
That said, not every early-lost baby tooth causes problems. Front baby teeth that fall out a few months early rarely matter much. Back baby molars lost at age four can matter quite a lot. We assess on a case-by-case basis.
Structural differences (and why they matter for cavities)
Baby teeth aren’t just smaller adult teeth. They’re structurally different in ways that affect how they decay and how they’re treated.
The enamel on a baby tooth is thinner, around half the thickness of adult enamel. The dentine underneath is also softer. This means decay can travel through a baby tooth much faster than through an adult one. A small cavity that might take two years to reach the nerve in an adult tooth could reach it in six months in a baby tooth.
That speed is one of the main reasons we encourage six-monthly check-ups for children. By the time a parent notices a hole, the decay has often already gone deep.
The pulp chamber (where the nerve lives) is also proportionally larger in baby teeth, sitting closer to the surface. This is why even small-looking cavities in a child’s mouth can cause significant toothache.
Eruption timeline at a glance
The exact timing varies by child, but the table below shows the typical pattern:
| Tooth | Baby tooth erupts | Baby tooth lost | Adult tooth erupts |
|---|---|---|---|
| Lower central incisors | 6 to 10 months | 6 to 7 years | 6 to 7 years |
| Upper central incisors | 8 to 12 months | 6 to 7 years | 7 to 8 years |
| Lateral incisors | 9 to 16 months | 7 to 8 years | 7 to 9 years |
| First molars | 13 to 19 months | 9 to 11 years | 9 to 11 years (premolars) |
| Canines | 16 to 23 months | 10 to 12 years | 9 to 12 years |
| Second molars | 23 to 33 months | 10 to 12 years | 10 to 12 years (premolars) |
| First adult molars | (none) | (none) | 6 to 7 years (“6-year molars”) |
| Second adult molars | (none) | (none) | 11 to 13 years |
| Wisdom teeth | (none) | (none) | 17 to 25 years (or never) |
The first adult teeth to come through are usually the lower central incisors and the “6-year molars” at the back. The 6-year molars often come through unnoticed because no baby tooth falls out, and parents are sometimes surprised to learn their six-year-old already has adult teeth at the back. Worth knowing: those 6-year molars are particularly vulnerable to decay because they’re often missed during brushing.
What to do about a chipped or knocked-out baby tooth
If a baby tooth is knocked completely out, the rule is simple: do not try to put it back in. Re-implanting a baby tooth can damage the developing adult tooth underneath. Bring the child to the dentist within 24 hours, but the tooth itself stays out.
If a baby tooth is chipped, broken, or pushed inward (intruded) after a fall:
- Calm the child first, comfort takes priority
- Rinse the mouth with warm water
- Use a cold compress on the cheek for swelling
- Phone the dentist the same day if possible, the next morning at latest
- Take a photo of the tooth, this helps us assess before you arrive
An adult tooth knocked out is a different matter entirely, and it’s a genuine emergency. The NHS has clear guidance on knocked-out teeth that’s worth bookmarking, particularly if you have a child who plays contact sports or rides a bike. With an adult tooth, every minute counts and re-implantation may be possible if you act quickly.
The mixed dentition phase: when things get interesting
Between roughly six and twelve, your child will have a constantly shifting mix of baby and adult teeth. This is the phase where:
- Adult teeth often erupt before the baby teeth above them have fallen out (giving “shark teeth” rows briefly)
- The new adult front teeth can look enormous and crooked, this is normal and usually corrects as the rest come through
- Crowding may become apparent and orthodontic referrals start to be considered
- Mouthguards become more important, particularly for football, rugby, and hockey at school
In our experience, this is also the phase when parents get worried about how their child’s teeth look. Most of those worries resolve naturally as more adult teeth come through. We’ll always tell you honestly if a referral to an orthodontist is sensible, and just as honestly if it’s worth waiting another year.
One point parents often ask about: should you keep the baby teeth that fall out? The honest answer is, do whatever feels meaningful to your family. There’s no clinical reason to keep them, but plenty of parents pop them in a small box for sentimental reasons. The tooth fairy tradition continues to thrive. The only practical caveat is that a saved baby tooth dries out and becomes brittle within a few weeks, so if you do keep them, expect them to look quite different to how they did when they came out.
Booking a check-up for your child
Whether your child is still cutting their first molars or is mid-way through the mixed dentition phase, regular check-ups make a real difference. Six-monthly visits catch small issues before they become big ones.
To book a check-up with our team at Westcliff Dental Practice, please call 01702 344 500. We’ll find a time around the school day and walk you through what’s normal for your child’s stage.
