The combination of increased use of fertility treatments and rising maternal age has caused a doubling in the rate of twin births in developed countries over the last 40 years, according to a recent study.
In England and Wales, the rate increased from 9.9 twins per 1000 births in 1975 to 16.1 in 2011, while similar increases were reported for the US (from 9.5 to 16.9), Germany (from 9.2 to 17.2) and France (from 9.3 to 17.4).
The increase is due to a higher incidence of fraternal twins, which arise when two eggs are released at the same time and fertilised by two different sperm. This event occurs more frequently in older women, and the authors think this partially accounts for the recent increase in twin births in developed countries.
The other reason for the increase, according to the study, is a 'sharp rise' in fertility treatments. In the early years of IVF it was commonplace to implant three or more embryos, resulting in high multiple birth rates by the year 2000. Ovarian stimulation by hormone treatments also increases the likelihood of two eggs being released at the same time.
'We're not sure if the rate will continue to rise, but the data is increasingly seen as a public health crisis', Gilles Pison, study co-author from the French National Institute for Demographic Studies, told the Independent.
In the report, published in Population and Development Review, the authors drew attention to the dangers associated with twin births compared to singletons: '[Twin babies] have lower birth weight, more complications at birth, and are more often born premature – all of which are associated with many long-term health problems.'
Multiple births also pose a risk to the mother, with an increased risk of health issues such as pre-eclampsia and postpartum depression.
The authors estimate the effect of fertility treatments on increased twinning rates to be on average 'about three times greater than the effect of delayed childbearing'. To address the high percentages of multiple births following medically assisted reproduction, many countries have introduced guidelines encouraging the transfer of fewer embryos following IVF.
Reflecting these changing practices, researchers observed a decline in multiple births in recent years in around a quarter of the countries studied. In the majority of countries however, including England and Wales, twin births have continued to rise.
The authors think this increase may reflect the increasing number of women turning to fertility treatments and the continuing trend toward later motherhood.
'The number of treatments continues to increase, and the average age of women undergoing these treatments is rising,' the team write.
They suggest that better data should be maintained on the age and nationalities of women having twin births in order to keep track of the impact of policies and practices around medically assisted reproduction.