How many twins are there in the us

That is, in part, because assisted reproductive technology procedures have changed. For example, the number of embryos implanted during IVF has a direct relationship with the number of multiple births. So, implanting two or even three embryos increases the chance that at least one of them will grow and develop, but simultaneously ups the risk of a plural pregnancy.

In , three or more embryos were implanted in two-thirds of IVF cycles. During the s, though, the Society for Assisted Reproductive Technology and American Society for Reproductive Medicine published guidelines that recommended fewer embryo transfers, particularly for younger women.

So, from until , one study found that the average number of embryos transferred had fallen from 2. In 10 percent of cases, the prospective parents chose to go with a single embryo transfer, too, which was unheard in earlier decades. It is now The birth rate for triplets and other higher-order multiples rose dramatically.

But it has slowed since Many factors are linked to having a multiple pregnancy. Naturally occurring factors are:. A family history of multiple pregnancy raises the chances of having twins. Older age. Women older than 30 have a greater chance of multiple conception. Many women today are waiting to have children until later in life. They may have twins as a result.

Past pregnancies. Having 1 or more previous pregnancies, especially a multiple pregnancy, raises the chances of having multiples. Twins are also more likely to survive now, thanks to medical advances. Most of the increase has been in fraternal or non-identical twins, who develop from separate eggs and sperm.

The rate of identical twins , caused by an embryo splitting in two in the first few days after fertilisation, has stayed about the same. Growing use of fertility treatments is probably the biggest factor behind the rise in twin births in high and middle-income countries, says Monden. Women may take hormones to stimulate egg production, which can lead to them releasing two eggs at once. Twins may be more common today than at any time in history, according to the first comprehensive survey of twin births around the world.

Researchers analysed records from more than countries and found a substantial rise in twin birthrates since the s, with one in 42 people now born a twin, equivalent to 1. According to the study, the global twin birthrate has risen by one-third, on average, over the past 40 years.

Twins are born naturally when a fertilised egg spontaneously splits in half, leading to identical twins, or when the mother releases two eggs at once that are both fertilised, producing non-identical twins.