Your child brushes, sweets are not an everyday thing, and the dentist has just found cavities anyway.
Cavities form when germs on the teeth turn sugar into acid more often than saliva and fluoride can repair the enamel. How often a child has sugar matters, not only how much, and so does the tooth itself: teeth that formed with weaker enamel decay more easily. It is usually several things at once, not one mistake.
Some of these causes are in the tooth, and nothing you did put them there. The ones in the daily routine can change, starting this week.
This page covers what actually causes cavities in kids, why brushing alone does not always stop them, what the evidence says about milk and night feeds, whether teeth can be "weak", and what changes your child's plan.
Is it normal for kids to get cavities?
It is common. A 2022 review that pooled 71 studies of 69,330 Indian children found decay in 46.9% of children under six: close to one child in two.
Common does not mean it can be left alone. The American Academy of Pediatric Dentistry (AAPD) notes that decay in young children raises the risk of new decay in both the milk teeth and the adult teeth. Why a cavity in a tooth that will fall out is still worth treating is set out in do milk teeth even matter.
What actually causes cavities in kids?
Four things have to meet.
Germs
They live in every mouth, and they turn sugar into acid.
Sugar
The fuel. Each time it reaches the teeth, the germs make acid.
The tooth
Some teeth form with weaker enamel and decay more easily.
Time
How often sugar arrives, and how long it stays on the teeth.
Cavities form when the germs on the teeth turn sugar into acid more often than saliva and fluoride can repair the hard outer layer of the tooth (the enamel). Researchers describe tooth decay as "sugar-driven": the germs are needed, but they live in every child's mouth, so they are not the part a family can remove. Sugar is the part a family can change.
Amount matters too. A review that informed the World Health Organization's sugar guideline found a link between more sugar and more decay in 42 of the 50 studies in children it looked at.
Usually, it is several of these at once, not one mistake.
Why does my child have cavities even though we brush?
Brushing matters, but it does not cover everything. These are the four things it does not cover.
1. How often sugar arrives
The AAPD counts sugary snacks or drinks between meals more than three times a day as enough, on its own, to put a young child at high risk. Drinks count: juice, sweetened milk, or a bottle or sipper cup of something sweet used through the day.
Parents often ask whether sugar can be added to a child's milk. The AAPD advises no added sugar before the age of two, recommends plain milk for children under five, and lists flavoured milks as a source of added sugar.
2. Sugar close to bedtime
Studies keep finding a link between sugary food or drink near bedtime and decay. A 2018 review of 18 studies rated the evidence as very low certainty, but all seven studies in preschool children pointed the same way.
3. The toothpaste
Fluoride is the main protection. In milk teeth, fluoride toothpaste prevented more decay than toothpaste without fluoride, at moderate certainty, in a Cochrane review. A fluoride-free toothpaste removes that protection.
The AAPD recommends a toothpaste with 1000 ppm of fluoride, twice a day: a smear under the age of three, a pea-sized amount from three to six, with little or no rinsing afterwards. The ppm is printed on the tube.
4. Who holds the brush
Young children cannot yet clean every surface on their own. The AAPD recommends that a parent brushes, or helps, for preschool children. More on routines is in the brushing, food and habits articles.
Does milk or breastfeeding at night cause cavities?
This is the question parents ask most carefully, so here is what the evidence says, including where it disagrees.
- The starting point. The World Health Organization recommends breastfeeding exclusively for the first six months and continuing up to two years of age or beyond.
- The first year. Breastfeeding up to 12 months is linked to a lower risk of decay: a 2015 review of 63 papers found about half the odds.
- After 12 months. Studies find more decay with frequent or night-time breastfeeding. A 2024 review of 31 studies found night-time breastfeeding linked to more than twice the risk. The newest review, of 25 studies and 19,681 children, found a clear rise only after 24 months, not between 12 and 24 months.
- What the studies could not separate. Feeding, sugar in the rest of the diet and brushing often go together, and the 2015 review could not pull them apart.
- Bottles. A 2026 review of 12 studies found a bottle in the first year was not linked to decay; the link rose after the first year and with night bottles, at low to very low certainty.
- Milk itself. Plain cow's milk and breast milk are less likely to cause decay than sugar solutions in animal studies. The AAPD lists frequent or night-time breastfeeding beyond the first year among the things associated with decay.
This page does not tell anyone to stop breastfeeding. The levers with the clearest support are the ones above: no added sugar, fluoride toothpaste twice a day, and a first dental visit early enough for the dentist to look at your child's own risk.
Are cavities genetic, or does my child have soft teeth?
Parents often call it "soft teeth" or "weak teeth". Some children's teeth do form with defects in the enamel, and those teeth decay more easily: a 2017 review of 16 studies of milk teeth found about three times the odds of decay. Nothing about brushing made the enamel form that way.
Genes play a smaller part in young children than many parents fear. In twin studies in Australia (345 twins, assessed at six) and in Chennai, identical twins were no more alike in their decay than non-identical twins, which points to the environment. Across a whole lifetime, a Swedish study of up to 41,678 twins found that genes explained a larger share.
Can cavities pass from parent to child?
The germs can. A 2015 review of 36 studies found that mothers pass decay-causing germs to their children.
In trials, treating mothers' own teeth to stop that made a very uncertain difference to children's decay, while advice to families on food and feeding probably lowered it. So the evidence points to sugar and fluoride as the levers, not to stopping kisses or shared spoons. If your own teeth have decay, it is worth getting them checked too.
Why does my child keep getting cavities, even after fillings?
A filling repairs a hole. It does not remove the cause. In the AAPD's words, treating the cavities alone "does not stop the disease process". A new cavity after fillings is a sign the causes are still there.
What changes is the plan around the fillings. The AAPD sorts children under six into low, moderate and high risk, and the plan changes with it.
| Part of the plan | Low risk | Moderate risk | High risk |
|---|---|---|---|
| Check‑ups | Every 6 to 12 months | Every 6 months | Every 3 months |
| Fluoride at the clinic | Not listed | Every 6 months | Every 3 months |
| Chalky white patches | Watched | Watched closely | Watched closely |
| Cavities | Not listed | Filled if open or growing | Silver diamine fluoride (SDF) to stop them; filled if open or growing; temporary fillings until permanent ones are possible |
Not every mark needs a drill.
The AAPD notes that many early lesions do not progress, and that decay can stop without treatment. At moderate and high risk, a chalky white patch is watched closely, not drilled. Cavities that are open or growing are filled, and the causes are worked on at the same time.
These are fair questions to ask at the next visit:
- What is my child's risk level, and what is driving it?
- How does the plan change if those things change?
- Is this mark still active, or has it stopped?
How fillings, caps and SDF work for children is on the treatment page.
When is it worth booking a visit?
You do not need to book an appointment because you read this page. These are the things worth acting on.
- Your child has not had a dental check yet. The AAPD recommends a first visit within six months of the first tooth coming through, and by the first birthday.
- You can see a chalky white line near the gum on the upper front teeth. Lift your child's upper lip and look. It is one of the earliest signs of decay, at the stage when it can still be reversed.
- A tooth hurts, or the gum beside it is swollen. Book soon.
- New cavities keep appearing after fillings. Ask about the risk level, not only the next filling.
Tooth Tales runs a programme built around exactly this decade: First Decade. No Decay. No Delay.