Advances in fertility treatment and a trend toward later-life childbirth are fuelling a boom in twins in the United States, where one in 30 babies is a twin, according to US data.
The number of twins doubled in 2009 compared to 1980, rising from 68,339 to over 137,000 births, the National Center for Health Statistics' data brief on three decades of twin births in the United States said on Wednesday.
The overall birth rate of twins surged 76 per cent over the past 30 years, going from 1.9 per cent of all births to 3.3 per cent.
That means one in every 30 US babies is a twin, compared to one in 53 in 1980.
Over the past three decades, twin birth rates rose in every state, by nearly 100 per cent among women aged 35-39 and more than 200 per cent among women aged 40 and over, it said.
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Women in their 30s tend to have higher rates of "spontaneous twinning", or becoming pregnant with twins without the use of fertility therapy, than younger women.
But older mothers are not solely responsible for the surge in twin births, making up only about a third of the rise in the past 30 years.
The rest of the increase is likely due to the mounting use of infertility treatments, such as in vitro fertilisation and ovulation stimulation medications, which have grown more common in women 30 and over since the 1980s.
The rate of twin births doubled among white mothers in the past 30 years, rose by about one-half among black mothers and by one-third among Hispanic mothers.
While twins may not be as novel as they once were, the United States might be a very different place without such advances against infertility, with 865,000 fewer twins born in the United States over three decades, the report said.
