Multiple births are more common than ever before. In many countries, the number of twin and other multiple births started to rise rapidly from the mid-1970s or early 1980s, without discontinuity for decades. In the United States, France, and Germany, the twinning rate rose from about 9.5 twin births per 1,000 deliveries in 1975 to more than 17 in 2015. The increase over the same period was even sharper in Spain (from 8.7 to 22.1) and Greece (from 9.5 to 26.0). This remarkable trend has been driven by two main factors: higher mean ages at childbearing – as the probability of a multiple pregnancy increases with the mother’s age – and the widespread use of medically assisted reproduction (MAR).
While having multiples may be a joy for many parents, it is more challenging than having a single child. Multiple children and their mothers have greater health risks during pregnancy, delivery, and even the years afterwards. The boom in multiple births has consequently become a public health concern.
To examine the frequency of twin and other multiple births over time and across countries, Catalina Torres (French Museum of Natural History [MNHN] and French Institute for Demographic Studies [INED]), Arianna Caporali (INED), and Gilles Pison (MNHN and INED) have developed the Human Multiple Births Database (HMBD). This open-access database provides statistics on the number of deliveries by multiplicity in 25 countries, from the earliest to the most recent year for which national vital statistics data are available. These data are easy to explore with interactive graphs and tables on the HMBD website (www.twinbirths.org).
Using the HMBD, the authors observed that after decades of sustained increase, the twinning rate peaked and has even started to decline in numerous countries. The forerunners are the Nordic countries (Finland, Norway, and Sweden), the Netherlands, as well as Australia and New Zealand, which experienced a peak in the twinning rate in the late 1990s and early 2000s. France, Germany, Spain, and the United States experienced a later peak (in the mid-2010s). Greece showed no sign of a peak by 2019, when the twinning rate reached 25.7 twin deliveries per 1,000 total deliveries.
Although the increase in the age at childbearing has played a central role in that of multiple births, country differences in the twining and the multiple rates, the speed of changes, and their timing are linked to variations in MAR use and practices. In the early days of artificial reproduction technologies (ART) in the late-1970s and the 1980s, transferring multiple embryos simultaneously was a common practice to enhance the chances of successful treatment, despite a higher probability of a multiple pregnancy. Technological improvements have made single embryo transfers increasingly successful. Accordingly, starting in the late 1990s, countries have gradually implemented regulations or guidelines to limit the number of embryos transferred. Although the share of ART treatments involving a single embryo transfer has risen, multiple embryo transfer remains common in certain countries.