A simian crease, now usually called a single palmar crease, is one line running straight across the palm where most hands have two separate lines. For most people who have one, it is simply the way their hand formed and says nothing about their health. Its reputation comes from being much more common in some genetic conditions, so it matters only when it shows up alongside other signs.
What the Single Crease Looks Like
A typical palm has three main creases. One curves around the base of the thumb, and two run roughly across the palm.1PubMed Central. Improved analysis of palm creases Those two crossing lines usually stay separate, one starting near the index finger side and the other near the little finger side.
In a simian crease, the two crossing lines are fused into a single crease running the full width of the hand.2Consultant360. Single Transverse Palmar Crease It can appear on one hand only or on both.3International Journal of Contemporary Pediatrics. The enigma of the simian crease: case series with the literature review The single line usually runs from the little finger edge toward the index finger side, in roughly the place the two separate lines would have covered.
The old name came from a supposed resemblance to the palms of monkeys and apes. Many doctors now avoid it because it can sound insulting, though you may still see “simian crease” online or in older medical notes.
Some patterns look similar but are not a true single crease. One is a lower crossing line that stretches all the way across the palm while the upper line stays separate.4International Journal of Morphology. Prevalences of Simian, Sydney and Suwon Creases and their Association with Each Other, Body Sides, Handedness, Sex and Anomalies/Diseases/Syndromes in a Population of Central India The test is whether the upper and lower lines have truly merged, not whether one of them is long or deep. Some palms show two lines joined by a short connecting line. That in-between pattern is usually not counted as a classic single crease.
To check a palm, open the hand flat in good light and look at the lines below the fingers. Count whether there are two crossing lines or one unbroken line. On a baby, gently opening the hand makes the creases much easier to see.
How Common It Is in Healthy People
About 1 in 30 newborns has a single palmar crease.5Consultant. Single Transverse Palmar Crease Estimates suggest around 10% of people have one on one hand and 5% on both hands.6International Journal of Contemporary Pediatrics. The enigma of the simian crease: case series with the literature review Studies of particular groups give different numbers. Among healthy medical and dental students in Ethiopia, about 6% of palms had one.7PubMed Central. Prevalence of the Different Types of Palmar Creases Among Medical and Dental Students in Addis Ababa, Ethiopia In a population in central India the figure was about 14%.8International Journal of Morphology. Prevalences of Simian, Sydney and Suwon Creases and their Association with Each Other, Body Sides, Handedness, Sex and Anomalies/Diseases/Syndromes in a Population of Central India
Males are about twice as likely as females to have one.9MedlinePlus. Single palmar crease: MedlinePlus Medical Encyclopedia How often it appears also differs noticeably between ethnic and regional groups.10PubMed. Palmar creases in population studies
The Link to Down Syndrome and Other Conditions
Down syndrome is a genetic condition caused by an extra copy of chromosome 21. Around 60% of people with Down syndrome have a single palmar crease.11Consultant. Down Syndrome Other estimates put it above 63%.12International Journal of Contemporary Pediatrics. The enigma of the simian crease: case series with the literature review In one group of children with Down syndrome, the most common genetic type had it in about 80%, while milder mixed forms had it in about 25%.13Journal of Medical & Health Sciences Review. FREQUENCY OF SIMIAN CREASE IN VARIOUS GENOTYPES OF DOWN SYNDROME
In the mixed forms, only some cells carry the extra chromosome, and these often show fewer typical features, which can include the crease. This also means the overall percentages are averages across types, not a figure that fits every person.
The crease is also reported more often in conditions including Turner syndrome, Klinefelter syndrome, trisomy 13, cri du chat syndrome, congenital rubella and fetal alcohol syndrome.14Consultant360. Single Transverse Palmar Crease But a careful study of fetal alcohol spectrum disorders found no link with palm crease pattern.15PubMed Central. Palmar Creases: Classification, Reliability and Relationships to Fetal Alcohol Spectrum Disorders So some listed links are weaker than they look. Doctors treat it as one clue among many, not a test.16PubMed Central. Accuracy of the clinical diagnosis of Down syndrome
Being more common in Down syndrome does not make it a likely sign of the condition. Down syndrome is uncommon in the general population, while a single palmar crease is common among healthy people, so the great majority of people with the crease do not have Down syndrome. A feature can be frequent in a condition yet far more often found in people without it, simply because there are so many more of them. Even among people with Down syndrome, roughly four in ten lack the crease, so its absence does not rule the condition out.
Confirmation does not rest on the palm. A newborn exam looks at overall appearance, including muscle tone, facial features, the eyes and the heart. A chromosome blood test then shows whether there is an extra copy of chromosome 21 and which genetic type it is. In pregnancy, screening tests can estimate the chance of Down syndrome and diagnostic testing can confirm it.
When to Mention It to a Doctor
A single palmar crease on an otherwise healthy, normally growing child needs no testing. Palm and fingerprint patterns can help doctors only when combined with other findings.17JAMA Pediatrics. Dermatoglyphics and Syndromes
It is worth raising if your child also has low muscle tone, feeding trouble, unusual facial features, heart problems or delayed milestones. Those features together are what point doctors toward a genetic condition.18PubMed Central. Accuracy of the clinical diagnosis of Down syndrome If you already have a regular checkup coming, mentioning the crease there is enough. Describing what you have noticed, such as a single line on one or both palms, helps the doctor assess it quickly.
If the crease sits alone in a thriving child, the usual response is reassurance that it is a normal variation. Chromosome testing is considered only when several signs fit together.
An adult who has grown and developed normally does not need any tests because of a single palmar crease. On its own, it does not signal any added health risk later in life. Palm-reading claims about what the line says of personality or fate have no medical basis.
How Palm Creases Form Before Birth
Palm and finger creases form between about 8 and 13 weeks of pregnancy,19PubMed. Development of human palmar and digital flexion creases early in the first trimester, before many people have had a first scan. Palm creases are reliably present by 13 weeks.20PubMed. Development of human palmar and digital flexion creases Most form alongside small fetal pads on the hand, and largely not from the hand’s movements.21PubMed. Embryological development of human palmar, plantar, and digital flexion creases Because the pattern is set so early, conditions affecting early development can alter it, which is why a crease can be more common in certain genetic conditions.
A common belief is that creases are folds from gripping and flexing, but sucking a thumb or clenching a fist cannot turn two lines into one. The pattern stays the same through life, though lines can look deeper or fainter with age and skin changes.
Why It Was Named After Apes
The crease was named for resembling the palms of non-human primates.22International Journal of Morphology. Prevalences of Simian, Sydney and Suwon Creases and their Association with Each Other, Body Sides, Handedness, Sex and Anomalies/Diseases/Syndromes in a Population of Central India Apes and monkeys do have palm creases, but the typical human set of palm lines never occurs in other primates.23Biological Communications. Palmar flexion creases and finger linkage groups in New World monkeys – functional and evolutionary palmistry In apes, the course of palm creases depends on how the animal moves, such as swinging through branches or walking on its hands.24Anthropozoologica (or Moravské zemské muzeum / Acta Musei Moraviae, Scientiae biologicae – based on header “XLI/1-2”, but journal name standard applies. Let’s use Acta Musei Moraviae, Scientiae biologicae). Classification of Flexion Creases on Palms of Apes in Dependence of Locomotion Type The name suggests a closer link than the anatomy supports.
Interest in hand lines began with palm reading, which long claimed to read fate from the palm. It then moved into early medical genetics, largely through this crease’s link with Down syndrome.25Oxford Academic. The Disenchantment of Chiromancy: Reading Modern Hands from Palmistry to Genetics