About one-third of all twins worldwide are identical. The rate of identical twinning holds remarkably steady at roughly 4 per 1,000 births no matter where in the world you look, while fraternal twinning varies widely by region, age, and other factors. That consistent baseline is what makes identical twins the minority in most populations.
The Global Numbers
Between 2010 and 2015, the worldwide twinning rate was about 12 twin deliveries per 1,000 total deliveries, meaning roughly 1 in every 42 children born was a twin. Of those, identical twins accounted for about 4 per 1,000 deliveries. That puts identical twins at approximately 33% of all twin births globally.
In the United States, twins make up a larger share of births. CDC data from 2023 shows 30.7 twins per 1,000 live births, well above the global average. But because the identical twinning rate stays fixed at around 4 per 1,000, identical twins represent a smaller slice of American twin births, closer to 13%. The rest are fraternal. The higher U.S. twin rate is driven almost entirely by fraternal twins, largely because of older maternal age at birth and the use of fertility treatments.
Why the Identical Rate Never Changes
Identical and fraternal twins form through completely different biological processes, which explains why their rates behave so differently.
Identical twins start as a single egg fertilized by a single sperm. At some point in the first two weeks after fertilization, that embryo splits into two. Nobody fully understands what triggers this split. It doesn’t appear to run in families, and it isn’t influenced by the mother’s age, ethnicity, diet, or body type. This is why the rate stays flat at 4 per 1,000 everywhere on Earth, across all populations and time periods. It seems to be a random biological event.
Fraternal twins, by contrast, happen when a woman releases two eggs in the same cycle and both get fertilized by different sperm. This process is influenced by genetics (it does run in families, specifically on the mother’s side), maternal age, number of previous pregnancies, and ethnicity. Rates of fraternal twinning range from as low as 6 per 1,000 in parts of Asia to over 20 per 1,000 in parts of West Africa. Fertility medications and procedures like IVF have also pushed fraternal rates higher in many countries over the past few decades.
Why the Percentage Varies by Country
Since the identical rate is essentially locked at 4 per 1,000, the percentage of twins who are identical depends entirely on how many fraternal twins a given population produces. In countries with low overall twinning rates, identical twins can make up half or more of all twin pairs. In countries where fraternal twinning is common, whether due to genetics or widespread fertility treatment use, identical twins become a much smaller fraction of the total.
This is why you’ll see different numbers depending on the source. A study focused on Nigerian twins might report that only 10 to 15% are identical, while data from Japan might put the figure closer to 50%. Both are correct for their population. The one-third figure represents the rough global average.
How Twins Find Out if They’re Identical
Many twins grow up unsure whether they’re identical or fraternal, especially if they look similar. The old assumption that twins who shared a placenta must be identical (and those with separate placentas must be fraternal) turns out to be unreliable. About one-third of identical twins actually develop with separate placentas, depending on when the embryo splits.
DNA testing is the definitive answer. Modern genetic tests compare specific markers in each twin’s DNA and can determine zygosity with effectively 100% accuracy. In one study of over 100 twin pregnancies, DNA-based testing correctly identified every fraternal and identical pair without a single error. These tests are available commercially as simple cheek-swab kits, and newer methods can even determine zygosity before birth using a blood sample from the mother.
Fraternal Twins Are Becoming More Common
The share of twins who are identical has been shrinking over time, not because fewer identical twins are being born, but because fraternal twinning has surged. In the United States, the twin birth rate rose about 75% between 1980 and the early 2010s before leveling off. Almost all of that increase came from fraternal twins, driven by two trends: more women having children after age 30 (older mothers naturally release multiple eggs more often) and the widespread use of fertility treatments that stimulate ovulation.
The identical twinning rate, true to form, didn’t budge during that same period. So while identical twins were once closer to a third of all twins in the U.S., they now represent a noticeably smaller share. The biology behind identical twinning remains one of the more puzzling constants in human reproduction.