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Twins can seem almost mysterious. Two people may share a birthday, resemble each other closely, and sometimes appear so alike that strangers struggle to tell them apart. Yet science increasingly shows that twinning is far more complicated than simply producing two matching children. The latest U.S. birth statistics add another surprise. About 3 in every 100 babies born in 2024 were twins, but the national twin rate has actually been falling for a decade. At the same time, genetic research is revealing new clues about why fraternal twins occur and why even “identical” twins are never perfect biological copies.
About 3 in Every 100 U.S. Babies Born in 2024 Were Twins
In 2024, 109,195 babies in the United States were born in twin deliveries, according to the National Center for Health Statistics. The national twin birth rate was 30.1 twin babies per 1,000 live births. That equals about 3.01%, or roughly 3 twin babies for every 100 babies born.
That number reflects an important detail. The statistic measures babies born in twin deliveries, not the percentage of pregnancies involving twins. Saying that about 3% of babies were twins is therefore more accurate than saying 3% of pregnancies were twin pregnancies. That difference may sound small, but it matters when interpreting national birth statistics. One twin delivery generally adds two babies to the total number of live births, so the pregnancy rate and the baby-based birth rate are not interchangeable.
America’s Twin Boom Has Been Reversing
Twins may still feel common, but the national trend is moving downward. In 2014, the U.S. twin birth rate was 33.9 twin babies per 1,000 births. By 2024, it had fallen to 30.1 per 1,000. NCHS demographers Joyce A. Martin and Michelle J.K. Osterman reported that this represents an 11% decline over the decade. The actual number of babies born in twin deliveries also dropped, from 135,336 in 2014 to 109,195 in 2024. That means the recent story of twins in America is not one of continued growth. Twins remain familiar enough to be part of everyday life, but both their rate and their total number have been decreasing.
The Sharpest Decline Was Among Mothers 40 and Older
The national decline was not evenly distributed across age groups. Among mothers ages 40 and older, the twin birth rate dropped from 66.0 per 1,000 births in 2014 to 35.6 in 2024. That was a 46% decline. Twin rates also fell among mothers ages 30–34 and 35–39. Those groups experienced declines of 18% and 27%, respectively, according to the NCHS analysis. The numbers are striking because older mothers have long been strongly associated with higher twin rates in national statistics. Still, the data describe demographic change rather than explaining exactly why individual twin births happen.
Twin Rates Are Not Moving the Same Way Across Every Population
National averages can hide important differences. Between 2014 and 2024, the twin rate among non-Hispanic White mothers fell from 36.7 to 31.3 per 1,000 births. Among Hispanic mothers, it declined from 24.1 to 23.4. The pattern was different among non-Hispanic Black mothers. Their twin birth rate increased from 40.0 to 42.0 per 1,000 births during the same period. These figures describe population-level birth patterns, not a single biological explanation. Birth-certificate data can show that groups have different rates and trends, but those statistics alone cannot establish why the differences occur.
Scientists Have Found Several Genes Linked to Fraternal Twinning
The idea that one simple “twin gene” explains it does not match what researchers are finding. In 2016, Hamdi Mbarek and colleagues studied 1,980 mothers of spontaneous fraternal twins and 12,953 controls. They identified genetic variants near FSHB and within SMAD3 that were associated with spontaneous dizygotic, or fraternal, twinning.
In a replication dataset, each relevant risk allele was associated with a higher likelihood of a twin birth. The increase was about 18% for the FSHB-linked variant and about 9% for the SMAD3 variant. Research published in 2024 further expanded the genetic picture. Mbarek and colleagues identified four additional loci associated with dizygotic twinning: GNRH1, FSHR, ZFPM1, and IPO8. Together, the findings point to multiple genetic pathways rather than a single inherited switch. They also help explain why the familiar claim that twins “skip a generation” is much too neat for the science.
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Identical” Twins Can Have Different DNA
Monozygotic twins begin from the same fertilized egg, which explains why they are commonly called identical. But a 2021 whole-genome sequencing study led by Ruud van Dongen showed that their genomes can begin to diverge very early in development.
The researchers found that identical twins differed by an average of 5.2 early developmental mutations. About 15% of the twin pairs studied had a substantial number of these early mutations present in one twin but not the other. These changes can arise as cells divide during early development. So while identical twins begin with extraordinarily similar genetic material, their DNA does not necessarily remain perfectly matched.