In recent decades, twin births have nearly doubled and even tripled in some developed countries.
Fraternal twins have skyrocketed, while the rate of identical twin births remained the same.
Here in the U.S., according to a study published in Population and Development Review, twin births per 1,000 deliveries increased from 9.5 in 16.9 between 1975 to 2011.
In France, the number nearly doubled from 9.3 to 17.4.
Yet, in South Korea, the number of fraternal twin births nearly tripled, from 5.0 to nearly 14.6.
Researchers say the reason for the increase could be because women are choosing to have babies later in life, and 30-something mothers are more likely to have fraternal twins than younger moms in their twenties.
A second reason for the twin explosion in developed countries is because more mothers in those areas can afford fertility treatments, which typically increase the risk of having more than one baby.
However, the new numbers come with some risks.
As the Atlantic reports, the researchers say twins come with a higher risk of premature birth, stillbirth, lower birth weights, and infant mortality.
Their mothers, meanwhile, have a higher risk of gestational diabetes, pre-eclamsia, and postpartum depression.
One important note, as Newser points out, fertility treatments, such as ovarian stiumulation or in vitro fertilization, may no longer be the culprit of multiple births that it once was, as doctors become more precise at transferring a single embryo, rather than several at once.