Decay in toddlers follows a recognisable pattern, usually beginning on the upper front teeth. The pattern reflects how those teeth are exposed rather than any difference in how they were formed.
Frequency matters more than quantity
Each time fermentable carbohydrate reaches the mouth, bacteria in plaque produce acid and the pH at the tooth surface falls for a period afterwards.
Saliva then buffers that acid and returns minerals to the enamel, but the recovery takes time. A new exposure before recovery finishes extends the acidic period rather than starting a new one.
This is why a drink sipped slowly over an hour affects teeth differently from the same drink taken at once, and why frequency features so heavily in preventive advice.
Sleeping with a bottle removes the defence
Salivary flow drops substantially during sleep, and with it goes both the buffering capacity and the physical clearance that normally washes residue away.
Liquid left pooling around the upper front teeth in those conditions sits in contact with enamel for hours with almost no protection operating.
The lower front teeth are frequently spared because the tongue covers them during suckling, which is why the pattern of affected teeth is so distinctive.
Young enamel is more vulnerable
Primary teeth have thinner enamel and dentine than permanent teeth, so a lesion has a shorter distance to travel before reaching the pulp.
Enamel also continues maturing after eruption, drawing minerals from saliva, which means recently erupted surfaces are more porous than they will later be.
Together these factors explain why decay in very young children can progress faster than a parent expects between one check and the next.
Bacteria arrive from close contacts
A newborn mouth does not contain the species most associated with decay. They are acquired, commonly from caregivers through shared spoons, cups and other close contact.
Earlier colonisation is associated with a higher likelihood of decay later, which is part of why the oral health of caregivers is discussed at all.
This is a matter of transmission rather than blame, and the practical measures follow from understanding the route.
Why early examination is recommended
The earliest visible sign is usually a dull white line along the gum margin, which is demineralised enamel rather than a cavity, and it can still remineralise.
By the time a brown or broken area is obvious, the process has advanced considerably, and options are narrower and more involved.
Guidance in most countries recommends a first dental visit around the time the first teeth appear, and any white or dark mark on a child's tooth is a reason to seek assessment.