So, is ART responsible for the rise in twin births? And if so, why is it not affecting the number of triplet-plus births? Turns out, the answer may be pretty simple. According to the CDC, improvements in ART technologies have sought to reduce the number of higher-order multiple births, in line with guidance from the American Society for Reproductive Medicine, because those babies face a higher risk of death. As the CDC said: “In 2010, the latest year for which mortality data are available, twins were more than 4 times, triplets 10 times and quadruplets more than 20 times as likely to die in infancy.”

But there’s one more piece to the puzzle. I had assumed, when I looked at who the mothers of those multiple births were, I had evidence for the ART assumption: They were much older. What I didn’t realize though was that even without treatments, older women are naturally more likely to have a multiple birth because they are more prone to producing multiple eggs in a cycle. So, part of the reason why twins are on the rise is because more women are having children at a later age. The CDC thinks that about one-third of the rise in twins is due to those natural factors — the rest attributable to the growing use and effectiveness of ART.