Skip to content
How It Works
Explore
Resources Ask a Question
Who It's For
Patients
Decode results, prep for appointments, weigh options.
Caregivers & advocates
Carry the research load for someone you love.
Providers & clinicians
Resources for patients in your practice.
Log In Ask a Question
PubMed This is a summary of 19 peer-reviewed journal articles Updated
Pediatric Dermatology · Large and Giant Congenital Melanocytic Nevus

Understanding Large and Giant Congenital Melanocytic Nevi (LCMN/GCMN)

At a Glance

Large and Giant Congenital Melanocytic Nevi (LCMN/GCMN) are rare birthmarks caused by a random NRAS gene mutation during fetal development. Doctors use a Projected Adult Size calculation to predict their final size, guiding lifelong monitoring for potential skin and neurological health risks.

Finding out your child has a Large Congenital Melanocytic Nevus (LCMN) or Giant Congenital Melanocytic Nevus (GCMN) can be overwhelming, but it is important to know that this was not caused by anything you did or did not do during pregnancy [1]. These marks are the result of a random change in the baby’s DNA that occurs long after conception [2].

Why This Happened

The vast majority of these nevi (about 80%) are caused by a somatic post-zygotic mutation in a gene called NRAS [3][1].

  • Somatic: This means the change is only in certain cells of the body (the pigment cells) and is not something that can be passed down to future children [1].
  • Post-zygotic: This means the mutation happened by chance in the embryo after the egg was fertilized [2].
  • NRAS Mutation: This specific gene normally tells cells when to grow. When it is “mutated,” it gets stuck in the “on” position, causing an overgrowth of melanocytes (the cells that produce pigment) in one area of the skin [1][4].

Understanding the Size and Scale

Because a baby’s skin grows as they do, doctors do not just look at the size of the mark today. Instead, they use a measurement called Projected Adult Size (PAS) [5][6]. This helps doctors estimate how large the nevus will be when your child is fully grown [7].

To calculate the PAS, doctors multiply the measurement taken at birth by a specific number based on where the nevus is located:

  • Head: Birth size ×\times 3
  • Trunk (Chest/Back): Birth size ×\times 2.8
  • Arms: Birth size ×\times 3.3
  • Legs: Birth size ×\times 4.5

A nevus is classified as Large if its PAS is expected to be 20 cm (about 8 inches) or greater [5][8]. It is often called Giant if the PAS is expected to exceed 40 cm (about 16 inches) [5][9]. While these marks are rare, GCMN is especially rare, occurring in roughly 1 in 20,000 to 1 in 500,000 births [10].

Common Patterns and Locations

Doctors often use the 6B system to describe where the nevus is located, as certain patterns are more common than others [11][12]:

  1. Bolero: Covers the upper back, including the neck and shoulders, resembling a short jacket.
  2. Back: Located on the back but does not include the buttocks or shoulders.
  3. Bathing Trunk: Covers the genital area and the buttocks.
  4. Breast/Belly: Located primarily on the chest or abdomen.
  5. Body Extremity: Located entirely on an arm or a leg.
  6. Body: A combination of patterns, often including both the “Bolero” and “Bathing Trunk” areas.

What This Means for Your Child

Most large or giant nevi are benign (non-cancerous) [13]. However, children with these marks require lifelong monitoring by a multidisciplinary team (a group of different specialists) for two main reasons [14][15]:

  • Skin Health: There is an increased risk of developing melanoma (a type of skin cancer) within the nevus or in “satellite” nevi (smaller spots elsewhere on the body) [16][17].
  • Neurological Health: In some cases, pigment cells can also grow in the brain or spinal cord, a condition called Neurocutaneous Melanocytosis (NCM) [8][6].

Management has shifted away from “removal at any cost” toward a focus on your child’s quality of life, psychological well-being, and careful clinical observation [18][19].

Back to Guide Home | Next: Understanding the Associated Risks: NCM and Melanoma

Common questions in this guide

Why did my child develop a large congenital melanocytic nevus?
The vast majority of these nevi are caused by a random genetic change, usually in the NRAS gene, that happens in the womb after conception. It is a somatic mutation, meaning it is not passed down from parents and was not caused by anything you did during pregnancy.
How do doctors determine if a nevus is large or giant?
Because a child's skin grows over time, doctors use a formula called Projected Adult Size (PAS). They multiply the birth measurement by a specific number based on the body location to estimate the final size. A nevus is large if the PAS is 20 cm or more, and giant if it exceeds 40 cm.
What are satellite nevi?
Satellite nevi are smaller pigmented spots that appear on other parts of the body, away from the main large birthmark. Your doctor will count these at birth, as their number can help determine the overall risk profile for conditions like melanoma.
What health risks are associated with LCMN and GCMN?
Children with these nevi have a higher risk of developing melanoma, a type of skin cancer. Some may also experience neurocutaneous melanocytosis (NCM), a condition where pigment cells grow in the brain or spinal cord, which is why lifelong monitoring is necessary.
Will the large nevus need to be completely removed?
Complete surgical removal is no longer the standard treatment at any cost. Modern management focuses on your child's quality of life, psychological well-being, and careful clinical observation rather than automatically attempting high-risk surgeries.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What is the Projected Adult Size (PAS) of my child's nevus based on their current measurements?
  2. 2.How many 'satellite nevi' did you count at birth, and how does that number affect the risk profile?
  3. 3.Which specialist(s) should be on our multidisciplinary care team (e.g., pediatric dermatologist, neurologist, psychologist)?
  4. 4.What are the specific signs of change in the nevus that I should look for at home?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (19)
  1. 1

    The retrospective molecular analysis of large or giant congenital melanocytic nevi in a group of Polish children.

    Wertheim-Tysarowska K, Szczygielski O, Seliga K, et al.

    Journal of mother and child 2021; (25(1)):19-24 doi:10.34763/jmotherandchild.20212501.d-21-00007.

    PMID: 34643354
  2. 2

    Genetic Abnormalities in Large to Giant Congenital Nevi: Beyond NRAS Mutations.

    Martins da Silva V, Martinez-Barrios E, Tell-Martí G, et al.

    The Journal of investigative dermatology 2019; (139(4)):900-908 doi:10.1016/j.jid.2018.07.045.

    PMID: 30359577
  3. 3

    Neurocutaneous melanocytosis (melanosis).

    Ruggieri M, Polizzi A, Catanzaro S, et al.

    Child's nervous system : ChNS : official journal of the International Society for Pediatric Neurosurgery 2020; (36(10)):2571-2596 doi:10.1007/s00381-020-04770-9.

    PMID: 33048248
  4. 4

    Melanoma arising in a Giant congenital melanocytic nevus: two case reports.

    Belysheva TS, Vishnevskaya YV, Nasedkina TV, et al.

    Diagnostic pathology 2019; (14(1)):21 doi:10.1186/s13000-019-0797-1.

    PMID: 30782194
  5. 5

    Cutaneous Melanoma Arising in Congenital Melanocytic Nevus: A Retrospective Observational Study.

    Caccavale S, Calabrese G, Mattiello E, et al.

    Dermatology (Basel, Switzerland) 2021; (237(3)):473-478 doi:10.1159/000510221.

    PMID: 33053549
  6. 6

    Melanoma on congenital melanocytic nevi.

    Alos L, Carrasco A, Teixidó C, et al.

    Pathology, research and practice 2024; (256()):155262 doi:10.1016/j.prp.2024.155262.

    PMID: 38518732
  7. 7

    Malignant peripheral nerve sheath tumor coexisting with congenital melanocytic nevus in an elderly man.

    Park JK, Sugita S, Soma T, et al.

    Pathology international 2016; (66(5)):309-10 doi:10.1111/pin.12392.

    PMID: 26897251
  8. 8

    Central Nervous System Involvement and Neuroradiological Imaging Insights of Neurocutaneous Melanocytosis in Congenital Melanocytic Nevi.

    Ruff C, Gohla G, Nägele T, Batra M

    Diagnostics (Basel, Switzerland) 2024; (14(21)) doi:10.3390/diagnostics14212345.

    PMID: 39518313
  9. 9

    Giant Congenital Melanocytic Nevi in a Pakistani Newborn.

    Mumtaz Hashmi H, Shamim N, Kumar V, Idrees S

    Cureus 2021; (13(5)):e15210 doi:10.7759/cureus.15210.

    PMID: 34178529
  10. 10

    Giant Congenital Melanocytic Nevus in a Chinese Newborn.

    Lin W, Zhou Y, Li R

    Clinical, cosmetic and investigational dermatology 2021; (14()):557-559 doi:10.2147/CCID.S308725.

    PMID: 34079321
  11. 11

    Patterns of distribution of giant congenital melanocytic nevi (GCMN): The 6B rule.

    Martins da Silva VP, Marghoob A, Pigem R, et al.

    Journal of the American Academy of Dermatology 2017; (76(4)):689-694 doi:10.1016/j.jaad.2016.05.042.

    PMID: 28325390
  12. 12

    Distribution Patterns (7B Rule) and Characteristics of Large Congenital Melanocytic Nevi: A Retrospective Cohort Study in China.

    Wang H, Wang W, Lu J, et al.

    Frontiers in medicine 2021; (8()):637857 doi:10.3389/fmed.2021.637857.

    PMID: 33681263
  13. 13

    Congenital Melanocytic Nevi.

    Nathan S, Smetona J, Naran S, Bauer BS

    Clinics in plastic surgery 2025; (52(2)):259-269 doi:10.1016/j.cps.2024.09.003.

    PMID: 39986887
  14. 14

    Congenital melanocytic nevi.

    Moustafa D, Blundell AR, Hawryluk EB

    Current opinion in pediatrics 2020; (32(4)):491-497 doi:10.1097/MOP.0000000000000924.

    PMID: 32692047
  15. 15

    Congenital melanocytic nevi and risk of melanoma.

    Pastore LM, Valentini R, Marghoob AA

    Clinics in dermatology 2025; (43(3)):378-384 doi:10.1016/j.clindermatol.2024.09.004.

    PMID: 39304091
  16. 16

    Prevalence, incidence density and standardized morbidity rate of melanoma among patients with congenital melanocytic naevi: a systematic review.

    Tan S, Hu H, Li G, et al.

    Clinical and experimental dermatology 2024; (49(8)):765-773 doi:10.1093/ced/llae056.

    PMID: 38380707
  17. 17

    Risk of melanoma in congenital melanocytic nevi of all sizes: A systematic review.

    Scard C, Aubert H, Wargny M, et al.

    Journal of the European Academy of Dermatology and Venereology : JEADV 2023; (37(1)):32-39 doi:10.1111/jdv.18581.

    PMID: 36149403
  18. 18

    Case Report: Prenatal ultrasound presentation of congenital melanocytic nevus syndrome.

    Shi Z, Sun T, Yin J, et al.

    Frontiers in pediatrics 2024; (12()):1466999 doi:10.3389/fped.2024.1466999.

    PMID: 39776646
  19. 19

    Updates in the Management of Congenital Melanocytic Nevi.

    Mologousis MA, Tsai SY, Tissera KA, et al.

    Children (Basel, Switzerland) 2024; (11(1)) doi:10.3390/children11010062.

    PMID: 38255375

This page provides educational information about large and giant congenital melanocytic nevi (LCMN/GCMN). It is not a substitute for professional medical advice, diagnosis, or treatment from your child's healthcare team.

Get notified when new evidence is published on Large/giant congenital melanocytic nevus.

We monitor PubMed for new peer-reviewed studies on this topic and email a short summary when something meaningful changes.