The Human Reproduction journal has recently published an article on worldwide twinning rates; Twin Peaks: more twinning in humans than ever before, Christiaan Monden, Gilles Pison, Jeroen Smits.
The International Council of Multiple Birth Organisations (ICOMBO) has provided a summary of this recent study based on the data of the Human Multiple Birth Database (HMBD) for developed countries.
Terms
𝑡𝑤𝑖𝑛𝑛𝑖𝑛𝑔 𝑟𝑎𝑡𝑒= (𝑡𝑤𝑖𝑛 𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑖𝑒𝑠 𝑡𝑜𝑡𝑎𝑙 / 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑑𝑒𝑙𝑖𝑣𝑒𝑟𝑖𝑒𝑠) × 1000
MAR – medically assisted reproduction. This includes IVF techniques and also simpler methods such as ovarian stimulation and artificial insemination.
The Data
• The twinning rate for 112 countries was collected for the period 1980 – 1985
• The twinning rate for 165 countries was collected for the period 2010 – 2015
• The twinning rates for these two periods were compared, to see if there was an increase or decrease in the rate between the two time periods
Results
• The twinning rate for monozygotic (identical) twins has remained at around 4 twin deliveries per 1000 deliveries everywhere in the world. The changes in rates are due to changes in dizygotic (fraternal) twinning rates.
• In both time periods, Africa had the highest twinning rates, and there no significant increase in twinning rate was observed between the two periods.
• Substantial increases in twinning rates, even doubling or more, could be seen in many countries in Europe, North America and East Asia.
• In 1980-85, the global twinning rate was 9.1 twin deliveries per 1000 deliveries. In 2010-15, it had increased to 12 twin deliveries per 1000 deliveries.
• In the period 2010 – 2015, the twinning rate for the world was 12.0 which means that one of every 42 children born on earth between 2010 and 2015 is a twin. There were about 1.6 million twins born in the world each year.
• In 2015, Asia and Africa were home to more than 80% of the world's twin deliveries.
• 42% of all twins born in 2015, were born in Africa. Sadly, these twins face a very high mortality rate.
Why has the twinning rate changed?
• The increased twinning rates were largely driven by reproductive and fertility choices of households and were initially concentrated in Europe and North America.
• MAR began in the wealthier regions of the world in the 1970s, spreading to emerging economies in the 1980s and later.
• The increasing age of the mother at birth has contributed to increased twinning rates in high-income countries.
• There may be other factors that influenced the change to twinning rates but no convincing evidence has been found yet.
Discussion
• The strong increase in the number of twin (and triplet etc) births due to MAR started to raise concerns in the 1990s in medical authorities and policymakers, because of the public health problems related to twin births. (twins are a high-risk group)
• As a result, many developed countries started to change their MAR regulations and clinical practices around 2000 – reduced numbers of embryos were transplanted and the focus was directed to the successful live birth of a singleton. It is likely that the 2015 twinning rate for these countries is an all-time high and rates may decrease in the coming decade.
• For example, in Europe the number of transfers of a unique embryo in IVF/ICSI was just over 10% in the late 1990s, but was just over 40% in 2017. Also, transfers of three or more embryos has declined steadily over time.
• It is known that substantial numbers of women travel to other countries for fertility treatments. These births may not be registered in their home country in the usual way, so their data doesn’t appear in these results.
• Accurate and detailed data on twin rates is important for forecasting the demand for health services given the health implications for twins and their mothers. This is particularly important in low-income countries, where mortality among twins is highest and care for women expecting twins is often inadequate by modern standards.
Citation: Christiaan Monden, Gilles Pison, Jeroen Smits, Twin Peaks: more twinning in humans than ever before, Human Reproduction, 2021;, deab029, https://doi.org/10.1093/humrep/deab029