What it looks like
The pectoralis major is the large fan-shaped muscle that covers most of the front of the chest. When it is absent or underdeveloped on one side, the chest on that side appears noticeably flatter or more hollow than the other side. The difference is usually most visible when the child raises their arms, presses their hands together in front of their chest, or is viewed straight-on in good lighting.
A related sign is the absence of the anterior axillary fold — the soft ridge of muscle at the front of the armpit. When the pectoralis major is present, this fold is visible when the arm is raised. When the muscle is absent, the fold is missing or significantly reduced, creating a characteristic flatness at the front of the armpit on the affected side.
In infants and young children, the asymmetry can be subtle and easy to miss, particularly in chubby babies where body fat obscures chest contours. It often becomes more noticeable as the child grows leaner through childhood and adolescence.
Why it happens in Poland anomaly
The pectoralis major develops during approximately the sixth week of embryonic development, when the subclavian and axillary arteries supply blood to the emerging chest wall tissue. In Poland anomaly, a brief disruption to this blood supply prevents the pectoralis major — and sometimes the pectoralis minor — from developing normally on one side.
This is the defining feature of Poland anomaly and is present in every case, regardless of whether there is also hand involvement, rib involvement, or breast involvement. The muscle may be entirely absent or only partially developed, ranging from complete absence to a thin, non-functional remnant.
Other conditions that can cause chest asymmetry
Missing or underdeveloped chest muscle on one side is highly characteristic of Poland anomaly, but it is worth knowing what else a physician might consider:
- Pectus excavatum — A sunken or indented sternum (breastbone) that affects the middle of the chest rather than one side. Both sides are usually involved, and the pectoral muscles are present.
- Pectus carinatum — A protrusion of the sternum (pigeon chest). Again, typically central and bilateral, with muscles present.
- Scoliosis — Curvature of the spine can create the appearance of chest asymmetry, but the pectoral muscles on both sides are present.
- Normal developmental asymmetry — Minor chest asymmetry is common and not medically significant. Poland anomaly involves actual muscle absence, not just minor variation in size.
When the pectoralis major muscle is genuinely absent on one side — rather than merely smaller — Poland anomaly is overwhelmingly the most likely explanation, especially when combined with any hand or rib differences on the same side.
Does it affect function?
Most people with Poland anomaly who are missing the pectoralis major live full, active lives. The pectoralis minor, serratus anterior, and other shoulder muscles compensate significantly. Athletes with Poland anomaly compete at high levels in a wide range of sports.
Some individuals notice limitations in specific movements — particularly pushing motions, overhead pressing, or movements that cross the body. These limitations vary considerably based on the degree of muscle absence and individual compensation. Many people with Poland anomaly are unaware of any functional limitation at all until they are specifically tested.
Chest reconstruction surgery, when pursued, is primarily done for cosmetic and psychological reasons rather than functional improvement.
When to see a doctor
You should seek an evaluation if you notice that one side of your child's chest consistently appears flatter or more hollow than the other, especially if the front of the armpit looks flat when the arm is raised. A pediatric plastic surgeon or pediatric orthopedic surgeon at a children's hospital can evaluate whether the pectoral muscle is absent and whether the full picture is consistent with Poland anomaly.
You do not need to wait for symptoms to worsen. An early diagnosis — even in childhood — is valuable for planning, for accessing resources, and for connecting with specialists before any decisions about treatment need to be made.
What specialist to see
A pediatric plastic surgeon is typically the most direct path. They are experienced with both the diagnostic features of Poland anomaly and the treatment options for chest and hand involvement. Most major children's hospitals have plastic surgery programs that see congenital chest wall conditions.
A pediatric geneticist may also be part of the evaluation, particularly to rule out overlapping conditions and to discuss recurrence risk.
Our specialist directory lists providers with Poland anomaly experience by state.
Next step: If you are seeing a flat or hollow area on one side of your child's chest with a missing armpit fold, ask your pediatrician for a referral to a pediatric plastic surgeon. Mention Poland anomaly by name. The diagnosis is typically straightforward once you are in front of the right specialist.