What is the reason for twins

Twin Peaks: There are more twins than ever before

Twin boom: More twins are being born worldwide than ever before - on a global average, one in 42 children is a twin today, according to a study. In Europe, North America and parts of Asia in particular, the proportion of twin births has increased by up to 70 percent over the past 40 years. The main reasons for this are later births and artificial insemination, as the researchers report.

Identical, dizygoti or semi-identical: Twins have fascinated us humans for thousands of years. Because those born at the same time share many characteristics - especially if they emerged from only one fertilized egg cell, as is the case with identical twins. Today these are also important helpers in research. Because they help to determine how high the genetic share of certain clinical pictures and characteristics is - or how a stay in space affects people.

One in 42 children today is a twin

But how often are twins born? Christiaan Monden from the University of Oxford and his colleagues have now examined this in the most comprehensive study to date. To do this, they evaluated the birth statistics of 165 countries - that corresponds to more than 99 percent of the world's population. Using this data, they compared the twin rates in the period from 2010 to 2015 with those in the period from 1980 to 1985.

The result: 1.6 million pairs of twins are currently born worldwide each year - this corresponds to twelve thousandths of all births. In other words, one in 42 children born today is a twin. "This means that the relative and absolute number of twins is higher today than it has ever been since the beginning of the record - it is probably an all-time high," says Moons.

Highest increase in Europe and North America

This can also be seen in the increase in twin births since the early 1980s: "In 74 out of 112 countries we have observed an increase of more than ten percent," the researchers report. Overall, twin births have increased by 42 percent worldwide, while the total number of births rose by just eight percent over the same period. As a result, the relative proportion of twins has increased by around 30 percent.

The increase is particularly high in Europe and North America. The proportion of twins there has increased by 60 to 70 percent since the 1980s. Germany, Austria, the Czech Republic, Switzerland, Greece and Spain are among the countries with the highest increases. But there is also a boom in twins in parts of Asia, including South Korea and Taiwan. Only in South America has the already low twin rate decreased slightly in the last few decades.

Dizygoti twins - children who arise from two egg cells fertilized at the same time - are primarily responsible for this increase. As the researchers found, the proportion of identical twins has remained the same - they occur in around four out of every thousand births.

In vitro fertilization and late mothers

But what is the cause of the twin glut? The research team attributes this primarily to artificial insemination: because women often have more than one fertilized egg implanted, the probability of multiple births increases. "The enormous changes in the global twin rates are largely due to the increased use of medically assisted reproduction," explain Monden and his team.

This relationship is also reflected over time: While these reproductive technologies were hardly widespread in the early 1980s, they are now established in most industrialized and emerging countries. As a result, the twin rates there also increased. And there is something else: in these countries women now have children much later than in the past. But the older the mother is at conception, the higher the chance of having twins.

Which of these two factors predominates differs from country to country: "In Spain, Greece and Singapore, the effect of artificial insemination is five to six times higher than that of later birth," the scientists report. In Germany, France and Sweden, on the other hand, reproductive medicine only plays twice as important a role as the increasing age of mothers.

Africa has a special position

The situation is different in Africa: there has always been the highest number of twin births in the world. The reason: The tendency to have twins is partly based on genetic predisposition, which is why twins often cluster in certain families. Such genes are particularly widespread in many populations in Africa. As a result, Africa has above-average twin rates.

The relative proportion of twins in Africa has not changed since the 1980s, as Moons and his team found. However, there are still more twins there today because the overall birth rate has increased. "In Africa, this increase is almost entirely due to population growth," says Moen.

In South America this has the opposite effect: because the genetic predisposition for twins there is only weak, especially in the indigenous population, the twin rates are naturally low. Population growth tends to reinforce this trend there. (Human Reproduction, 2021; doi: 10.1093 / humrep / deab029)

Source: European Society of Human Reproduction and Embryology

March 15, 2021

- Nadja Podbregar