One reason for the increased twinning rates seems to be the increase in assistive reproductive technologies. These include fertility-boosting medications and artificial insemination, but in-vitro fertilization (IVF) is the most popular. As the average age of a first-time mother has risen in recent years—in 2014 it was 26.3, compared to 22.7 in 1980)—older moms with reduced fertility have a greater need for fertility-boosting technologies in order to conceive.

Because of how IVF is done, multiple births, like twins or triplets, become more likely. If a couple is having trouble conceiving naturally, scientists can combine the parents’ extracted egg and sperm in the lab, then implant a fertilized embryo in the mother’s uterus. (More often than not, these embryos won’t take, so hopeful parents have to endure multiple rounds of expensive and emotionally exhausting procedures—the authors of a recent study in the Journal of the American Medical Association found that women were increasingly likely to conceive with each successive round of IVF, up until the ninth round.)