The incidence of twin births in the USA has declined to the lowest rate in a decade, the US Centres for Disease Control and Prevention (CDC) reports.
The rate of twin births has decreased by four percent in the last four years, after decades of steady growth since the 1980s. The report emphasises the importance of tracking birth rates due to the higher risk of pregnancy and birth complications affecting twins and their mothers.
'Though the majority of twins are healthy, the risks are higher for them,' Dr Dmitry Kissin from the CDC told the Wall Street Journal. 'The reduction of twins in this case is a very good thing.'
Numbers of twin births more than doubled between 1980 (68,339) and 2007 (138,961) – coinciding with the availability of IVF, and the societal trend towards women having their children later in life. As a proportion of total births twins went from 18.9 per thousand to 33.9 – a 67 percent increase.
However, since 2014, the rate has started dropping by an average of two percent a year. In 2018, the number of twins born (123,536) marked the lowest number reported since 2002.
The report, which used data from the National Vital Statistics System, showed that rates declined more with maternal age: falling by 10 percent for women aged 30 to 34, 12 percent for women aged 35 to 39 and 23 percent for women aged 40 and over. No changes were seen in birth rates among women under 30. Furthermore, twinning rates only fell significantly among white mothers, they remained stable among black and Hispanic women.
White women over 30 are known to make up the majority of patients at US IVF clinics, and multiple pregnancies have been a common result of fertility treatment, although this may now be changing.
'In vitro fertilisation has improved,' Dr Kevin Doody from the Centre for Assisted Reproduction, Texas, who was not involved in the report told NPR. He called IVF technology 'much more mature' and continued:
'As we became more aware of the health risks of multiple pregnancy and made efforts to reduce it with IVF, I think that's translated to a reduction in doing things that are risky from an ovulation induction or super-ovulation standpoint.'