About 1.6 million twins are born worldwide every year, meaning one in every 42 babies delivered is a twin. That rate has climbed steadily over the past four decades, rising by a third from roughly 9 to 12 twins per 1,000 births globally. The total number of twins alive today is harder to pin down precisely, but with twinning rates at historic highs for decades running, the living twin population numbers well into the hundreds of millions.
How the Global Twinning Rate Got Here
The rise in twin births is one of the most striking demographic shifts of the last 40 years. In the United States alone, the twin birth rate jumped 76 percent between 1980 and 2009, from 18.9 to 33.2 per 1,000 births. In practical terms, one in every 30 American babies born in 2009 was a twin, compared to one in 53 in 1980. If the 1980 rate had held steady, roughly 865,000 fewer twins would have been born in the U.S. over that period.
Two forces drove the increase. First, more people started having children later in life. Women over 30 are naturally more likely to release multiple eggs during ovulation. Twin birth rates nearly doubled for women aged 35 to 39 between 1980 and 2009, and more than tripled for women 40 and older. Second, the spread of fertility treatments played a major role. In the U.S. in 2015, about 17 percent of multiple births were linked to IVF, and another 29 percent to other fertility treatments like ovulation-stimulating medications.
Where Twins Are Most and Least Common
Twinning rates vary dramatically by region. Sub-Saharan Africa has long had the highest natural rates, around 15 fraternal twins per 1,000 deliveries. Europe and North America sit in the middle, historically around 7 per 1,000 before fertility treatments pushed those numbers much higher. East Asia has consistently had the lowest rates, roughly 2 to 4 fraternal twins per 1,000 deliveries. Japan’s twinning rate peaked around 11.5 per 1,000 in 2005, still well below Western countries.
The most remarkable outlier is Igbo-Ora, a community in southwest Nigeria, which records about 45 to 50 sets of twins per 1,000 births. That’s roughly four times higher than anywhere else on Earth. The Yoruba ethnic group in the region has a spontaneous fraternal twinning rate of 4.4 percent of all births, a pattern researchers have documented for decades. The cause isn’t fully understood, though local diet and genetics are both considered contributing factors.
Within the U.S., state-level rates also show real variation. In 2018, New Mexico had the lowest twin birth rate at about 25 per 1,000, while Connecticut hit 36.4 per 1,000.
Identical vs. Fraternal: Two Different Stories
Identical twins, where a single fertilized egg splits into two embryos, occur at a remarkably steady rate of about 4 per 1,000 births everywhere in the world. It doesn’t vary by ethnicity, geography, diet, or age. This consistency suggests identical twinning is essentially a random biological event.
Fraternal twins are the variable. They form when two separate eggs are fertilized at the same time, and their frequency is influenced by genetics, maternal age, and fertility treatments. Virtually all of the global increase in twinning over the past four decades comes from fraternal twins. When researchers talk about regional differences or the impact of IVF, they’re talking about fraternal twins.
Have We Reached Peak Twinning?
Researchers believe high-income countries have likely reached or passed their peak twinning rates. The fertility medicine field has increasingly shifted toward transferring a single embryo during IVF rather than multiple embryos, specifically to reduce twin and triplet pregnancies, which carry higher health risks for both mothers and babies. In the U.S., the pace of increase had already slowed to less than 1 percent per year by the mid-2000s, and twin birth rates began declining in many states between 2014 and 2018.
The picture in lower-income countries could go the other direction. As fertility treatments become more accessible in parts of Africa, Asia, and Latin America, twinning rates in those regions may still climb. The net global effect depends on whether declines in wealthy nations outpace potential increases elsewhere. For now, with 1.6 million twins arriving every year and the rate holding near historic highs, the world’s twin population continues to grow.