Identical twins begin as a single fertilized egg that splits in two, so they start with almost the same genetic recipe. But the same recipe doesn’t guarantee the same result, because looks depend on how genes are used and on everything that happens to the body before and after birth. Differences between identical twins are normal, and they tend to widen over a lifetime.
Differences That Start in the Womb
The single embryo splits sometime in its first two weeks, and the timing decides whether the twins get separate placentas or share one.1Zygote. The timing of monozygotic twinning: a criticism of the common model Twins who split later share a placenta and are linked through its blood vessels.2PubMed. Twin-twin transfusion syndrome and related monochorionic disorders: historical perspectives, current controversies, and evidence-based opportunities
In a condition called twin-to-twin transfusion syndrome, blood flows unevenly through those shared vessels from one twin to the other.3PubMed Central. Overview and Long-term Outcomes of Patients Born With Twin-to-Twin Transfusion Syndrome The twin giving blood can grow poorly and be born smaller, while the receiving twin’s heart can be strained.4PubMed Central. Overview and Long-term Outcomes of Patients Born With Twin-to-Twin Transfusion Syndrome
A birth-weight gap is not proof of unequal blood sharing, though. Twins with separate placentas can differ too, because each placenta and cord attachment on the uterine wall can supply nutrients a little differently. Each twin also sits in a different position with different room to grow. These small inequalities can affect size without any diagnosed syndrome, and only specialized scans can show whether blood is being shared unequally.
In the first years, a twin who was born noticeably smaller may stay a little lighter or shorter, which can make toddlers look less alike. The cause is early nourishment, not genes. Many smaller twins catch up partly or fully, but how much varies from pair to pair. Parents can ask the pediatrician to plot each twin on their own growth curve rather than comparing the two directly.
Shared-placenta pregnancies usually get more frequent scans to watch for unequal growth. Survivors of unequal blood sharing can have later developmental issues, so follow-up matters.5PubMed Central. Overview and Long-term Outcomes of Patients Born With Twin-to-Twin Transfusion Syndrome
Identical Twins Don’t Have Perfectly Identical DNA
Identical twins differ on average by about 5 small DNA changes that arose early in development.6PubMed. Differences between germline genomes of monozygotic twins These changes happen as cells copy themselves in the embryo, not because the parents passed on different genes.7PubMed Central. A characterization of postzygotic mutations identified in monozygotic twins With trillions of cell divisions over a lifetime, such copying errors keep adding up.8PubMed. Somatic point mutations occurring early in development: a monozygotic twin study
Most of these changes land in DNA that does nothing visible, so a typical one has no noticeable effect on looks. Occasionally a change falls in a gene that matters, or arises in only part of the body’s cells, and then a trait or health condition can differ between twins. That’s why identical twins aren’t perfect genetic copies, even though they remain far closer than ordinary siblings.
Twins can also differ in larger stretches of DNA that are duplicated or missing, even when they look alike.9PubMed Central. Phenotypically Concordant and Discordant Monozygotic Twins Display Different DNA Copy-Number-Variation Profiles These differences seem more common when twins differ visibly or in health.10Forensic Science International: Genetics Supplement Series. Genome-wide comparison of copy number variations in monozygotic twins11PubMed Central. Ontogenetic De Novo Copy Number Variations (CNVs) as a Source of Genetic Individuality: Studies on Two Families with MZD Twins for Schizophrenia
Sisters can differ in another way. Early in a girl’s development, each cell randomly shuts down one of its two X chromosomes, and all of that cell’s descendants keep the same choice.12PubMed Central. Skewed X inactivation in a female MZ twin results in Duchenne muscular dystrophy Because the choice happens after the split, each sister ends up with a different mix of cells running the X from her mother versus her father. If one X carries a faulty gene, the sister whose cells mostly use that X can show the condition while her twin stays unaffected.
“Identical” means the twins came from one egg, not that they are exactly alike. A visible difference doesn’t mean the hospital made an error or that the twins are really fraternal. Fraternal twins come from two eggs and share about half their variable DNA, like ordinary siblings. If parents are unsure which type they have, a DNA test comparing the two children, often done with a cheek swab, can settle it.
Chemical Switches on Genes Drift Apart With Age
Genes carry chemical tags that turn them up or down without changing the DNA itself. Differences in these tags between identical twins grow from early adulthood into old age.13PubMed Central. Epigenetic variation during the adult lifespan: cross-sectional and longitudinal data on monozygotic twin pairs
The same gene can be busy in one twin and quieter in the other, even though the DNA letters are identical. Lifestyle and plain chance both feed into the switches, so twins who live very similar lives can still drift apart a little.
This is also why childhood photos look more alike than recent ones. Young twins have had little time for differences to build, and they usually share a home, meals, and routines. Each decade then adds separate jobs, illnesses, moves, and habits.
Mirror-Image Twins
An estimated 25% of identical twin pairs show mirroring of small features, like one twin’s trait on the left and the other’s on the right.14PubMed. Mirror, mirror? An evaluation of identical twin mirroring in tooth crown morphology These are called mirror-image twins, and the pattern is thought to come from the egg splitting later than usual.15PubMed. Mirror, mirror? An evaluation of identical twin mirroring in tooth crown morphology
A hair whorl or cowlick can spin clockwise on one twin’s head and counterclockwise on the other’s. A birthmark, dimple, or mole can sit on the left cheek of one twin and the right cheek of the other. One twin may be left-handed while the other is right-handed, and small differences in tooth placement can appear on opposite sides. Mirroring doesn’t make twins less identical, and it involves small features, not flipped organs.
Sun, Smoking and Lifestyle Show on the Face
Twins with the biggest differences in visible aging also had the most different habits.16PubMed. How environment and lifestyle choices influence the aging process The twin with more sun exposure, more outdoor activity, and no sunscreen tends to look older, and the gap widens with age.17PubMed. Factors contributing to the facial aging of identical twins Smoking and sun also make dark, patchy skin spots more common.18PubMed. Factors contributing to the facial aging of identical twins Twins who skipped sunscreen or foundation had more facial wrinkles.19PubMed Central. Objective assessment of facial skin aging and the associated environmental factors in Japanese monozygotic twins Alcohol, stress, diet, exercise, illness, and medicines may contribute too.20PubMed. How environment and lifestyle choices influence the aging process
Weight matters in more than one way. Heavier body weight was linked to fewer dark skin patches.21PubMed. Factors contributing to the facial aging of identical twins Fat in the face fills hollows and softens lines, so a heavier twin may look smoother and a bit younger around the cheeks. A thinner twin can look more angular or older, even if healthier overall. A youthful look is not the same as better health, so a twin’s face isn’t a health score.
When One Twin Gets Sick and the Other Doesn’t
In twins where only one had lupus or myositis, the affected twin carried far more autoantibodies.22PubMed Central. Twins discordant for myositis and systemic lupus erythematosus show markedly enriched autoantibodies in the affected twin supporting environmental influences in pathogenesis That means something besides genes helps trigger these diseases. Twins also swap a few cells in the womb, and these borrowed cells may sometimes unsettle the immune system.23PubMed Central. Microchimerism in twins
Eyesight can split a pair too. Nearsightedness is shaped by environment as well as genes, so identical twins can need different glasses.24PubMed. Heritability of myopic refractive errors in identical and fraternal twins Poor visual input early in life may shape how the eye grows.25PubMed. Heritability of myopic refractive errors in identical and fraternal twins
Illness changes looks. Rashes, weight loss, or medicines such as steroids can change a face, and steroids in particular can cause rounding of the face over time. Even wearing glasses can make a pair look different at a glance.
A healthy twin should tell their own doctor about the other twin’s diagnosis, since family history matters in judging risk. The healthy twin can ask whether earlier checks or screening make sense, and the doctor can decide based on the specific condition.
Fingerprints, Face Scans and Telling Twins Apart
Identical twins never have the same fingerprints. Ridges form in the womb from signals in the growing fingertip, and the pattern depends on finger shape and the exact timing of skin growth. Pressure from the surrounding fluid and the twin’s position varies slightly for each baby.
Face software tells twins apart best with same-day studio photos and neutral expressions, erring only about 1 to 4 times in 100.26arXiv / IEEE. Distinguishing Identical Twins by Face Recognition In ordinary lighting, errors rose to roughly 12 to 21 in 100, and with photos a year apart, about 15 to 27 in 100.27arXiv / IEEE. Distinguishing Identical Twins by Face Recognition So a phone’s face unlock may open for an identical twin. If your twin could unlock your phone with their face, switch to a PIN or fingerprint lock.