What is driving changes in twinning in the developed countries? – JUMEAUX

In developed countries the twinning rate has increased tremendously (almost two-fold) since the 1970s. This increase is an important public health issue because twin babies are more fragile than singletons, due to their low birth weight, their tendency to be born premature, and complications at birth, which all have long term health implications. A better understanding of what is driving variations and trends in twinning is urgently needed.

A major contribution of JUMEAUX will be to compile a unique database containing annual twinning rates (i.e. the proportion of twin deliveries, expressed per 1,000 deliveries) for all countries for which reliable statistics exist, based on complete or nearly complete civil registration (mostly developed countries and a few developing countries). We will make the database available to the research community on the website of the French Institute for Demographic Studies (INED).

Using the database and other complementary information, we will examine the factors driving trends in twinning rates in developed countries. Three sets of interacting determinants are central: behavior (birth control, delayed childbearing, intermarriage), biology (maternal age, parity), and medical technology (assisted reproductive technology, oral contraception). We will assess their role in past and current trends and disentangle their effects.

In the past, the two most important factors were mother’s age and birth order. Beginning with a low level at puberty, the twinning rate increases steadily, peaks at age 37, then rapidly decreases, returning to a low level at menopause. The age-adjusted rate of twin births rises with birth order. These ‘parity’ variations, which are smaller than those related to age, have been interpreted as physiologically driven, but the underlying mechanism is still a mystery.

Two more behavioral factors have emerged since the 1960s: highly effective voluntary fertility control and assisted reproductive technology (ART). More widespread voluntary fertility control in a population lowers the twinning rate through a selection effect by fecundability: women who are predisposed to having twins, are decreasingly represented in birth orders higher than one as voluntary birth control becomes more prevalent. On the contrary, ART is associated with a greatly increased risk of multiple births.

So far, no study has assessed the influence of all these factors in one comprehensive project. JUMEAUX is able to do so by bringing together existing data on an unprecedented scale. JUMEAUX will answer important questions regarding trends in twinning rates in rich countries. To what extent is the increase in twinning rates since the 1970s due to an increase in the maternal age at birth and to what extent to the rise of ART and other fertility treatments? Will twinning rates continue to increase or will they reach a plateau or even start to decrease under the influence of changing ART practices? In which countries will developments occur, and what form will they take? What policies might be required to steer the developments in a specific direction?

On the basis of our measurements of levels and trends in twinning for (almost) all developed countries of the globe, and using the newest insights into the factors affecting twin births, the project, in a last step, will make forecasts up to 2050 based on different scenarios.