When does tooth O come in and fall out?
The lower baby central incisors usually come in at 6–10 months and fall out at 6–7 years (ADA). The AAPD chart gives 5–8 months to come in and 6–7 years to fall out.
In the ADA chart, they are among the first to come in, and they are among the first to fall out, along with the upper baby central incisors.
Tooth O is replaced by tooth 24, the lower left central incisor, which usually comes in at 6–7 years (ADA). Under the gum, tooth O starts to harden in the 4th month of pregnancy and is fully formed by about 18–24 months (AAPD).
Tooth O: often the very first tooth
These are usually the very first baby teeth, at about 6–10 months (ADA). The AAP also says the lower central incisors are the first to come in.
Teething. Babies may drool, chew more or get a rash around the mouth. The AAP says teething does not cause fever, diarrhea or excess crying, and warns that numbing gels with benzocaine and teething necklaces are not safe.
Born with a tooth. About 1 in 2,000 to 3,000 babies are born with a tooth (a natal tooth), most often a lower baby central incisor and usually in pairs. Most are normal baby teeth that came in early; fewer than 10% are extra teeth. One is removed if it is an extra tooth or very loose; if it doesn't get in the way of breastfeeding and causes no problems, no treatment is needed.
The same tooth on the right side, tooth P, has its own section on losing it, and the adult tooth coming in behind.
Where is tooth O?
Tooth O is one of the two bottom front teeth right in the middle of your child's mouth: the one on their left side.
"Left" and "right" always mean the child's own sides, so when you face your child, tooth O is on your right.
Right next to it, across the middle, is tooth P, the other lower baby central incisor. Behind it is tooth N, the lower left baby lateral incisor.
Its mirror twin on the right side is tooth P. The tooth in the same spot in the upper jaw is tooth F, the upper left baby central incisor.
Dental charts show the patient's right side on the left, so tooth O sits just right of the center line of the bottom row. The letters then drop to the lower jaw: lower left teeth run from K (the back molar) to O (the front tooth).
What does the lower baby central incisor do?
Baby incisors are the front teeth young children use to bite and cut soft food. Baby teeth matter even though they fall out: children need them to chew, speak and smile, and they help the adult teeth come in correctly.
How many roots does tooth O have?
One small root, which dissolves as the adult tooth grows underneath.
Common problems with lower baby central incisors
- Injuries. Worldwide, about 1 in 5 children (22.7%) injure at least one baby tooth. In baby teeth the most common injury is a tooth pushed out of position (a luxation) rather than a broken tooth.
- Cavities. The AAPD advises repairing decayed baby teeth when possible, rebuilding each tooth to its original shape so it keeps its place until the adult tooth is ready.
If a baby tooth is knocked loose or out in a fall, or your child has swelling, fever or pain, call your dentist. Never put a knocked-out baby tooth back in (Knocked-Out Tooth Rescue explains why). Swelling that makes it hard to breathe or swallow is an emergency: call 911. These are typical problems for this kind of tooth, not a diagnosis.
Tooth O in FDI and Palmer notation
| System | Tooth O is written | What it means |
|---|---|---|
| Universal (ADA, used in the US) | O | Baby teeth are lettered A–T around the mouth, starting at the upper right back molar. |
| FDI / ISO 3950 (international) | 71 | First digit 7 = lower left quarter (baby teeth use 5–8); second digit 1 = first baby tooth from the middle. Said "seven-one". |
| Palmer | ┌A (often typed LLA) | The ┌ bracket marks the lower left quarter; A = first baby tooth from the middle. |
Palmer stops at E. Palmer letters only run from A to E in each quarter, so an "O" can only be a Universal letter.
FDI baby teeth start with 5 to 8. The first digit 7 marks the lower left baby quarter, so FDI 71 can't be mixed up with an adult tooth.
Letters and numbers don't line up. O is letter 15 of the alphabet, but baby tooth O is replaced by adult tooth 24, not tooth 15.
All baby teeth in the lower left quarter
From the front of the mouth to the back:
| Universal | FDI | Palmer | Tooth |
|---|---|---|---|
| O | 71 | ┌A (LLA) | Baby central incisor (this tooth) |
| N | 72 | ┌B (LLB) | Baby lateral incisor |
| M | 73 | ┌C (LLC) | Baby canine |
| L | 74 | ┌D (LLD) | Baby first molar |
| K | 75 | ┌E (LLE) | Baby second molar |
The Tooth Number Chart & Converter converts any tooth between the three systems.
Tools for tooth O
- Teething Calculator: which baby teeth are due at your baby's age.
- Baby Teeth Tracker: log each tooth in and out.
- Kids Fluoride Calculator: how much toothpaste to use; the ADA says a rice-grain smear until age 3, then a pea-size amount.
- Tooth Fairy Calculator: what families give for a lost tooth.
- Dental Treatment Plan Decoder: plain English for treatment-plan shorthand like "#30 MOD comp".
Frequently asked questions
What tooth is letter O?
When does tooth O come in?
When does tooth O fall out?
Can a baby be born with tooth O?
Sources
- American Dental Association (MouthHealthy). Permanent Tooth Development (eruption chart). 2012. Accessed October 26, 2026.
- American Dental Association (MouthHealthy). Primary Tooth Development (eruption chart). 2012. Accessed October 26, 2026.
- American Academy of Pediatric Dentistry. Dental Growth and Development (chronology after Logan & Kronfeld 1933, adapted 2003). Accessed October 26, 2026.
- American Academy of Pediatric Dentistry. Management of the Developing Dentition and Occlusion in Pediatric Dentistry (Reference Manual of Pediatric Dentistry). latest revision 2024. Accessed October 26, 2026.
- World Health Organization. Oral health surveys: basic methods, 5th edition (tooth notation after ISO 3950). 2013. Accessed October 26, 2026.
- Nelson SJ. Elsevier. Wheeler's Dental Anatomy, Physiology and Occlusion, 11th edition. 2019.
- American Dental Association. ADA Dental Claim Form (tooth designation systems). Accessed October 26, 2026.
- Dentalcare.com (continuing education course CE500). An Overview of Dental Anatomy: Types of Teeth and their Functions. Accessed October 26, 2026.
- American Dental Association (MouthHealthy). Tooth Decay with Baby Bottles. Accessed October 26, 2026.
- American Academy of Pediatrics (HealthyChildren.org). Teething Pain Relief: How to Soothe Your Baby's Discomfort. updated July 9, 2025. Accessed October 26, 2026.
- Levin L et al. (International Association of Dental Traumatology). Dental Traumatology 36(4):309–313. IADT guidelines for the management of traumatic dental injuries: General introduction. 2020. Accessed October 26, 2026.
- Petti S, Glendor U, Andersson L. Dental Traumatology 34(2):71–86. World traumatic dental injury prevalence and incidence, a meta-analysis. 2018. Accessed October 26, 2026.
- Leung AK, Robson WL. Journal of the National Medical Association 98(2):226–228. Natal teeth: a review. 2006. Accessed October 26, 2026.
Last reviewed by the Dental Health Time editorial team, against the sources above. How we review.