With two major Hollywood couples expecting twins – Geoge and Amal Clooney, and Jay-Z and Beyoncé – you may be hearing a lot about multiple births these days. But just how common are twins?

Although the twin birth rate is at an all-time high, it’s still a fairly uncommon occurrence. According to the Centers for Disease Control and Prevention, 33 sets of twins are born per each 1,000 births in the United States. The likelihood of twins has increased steadily since 1980, when 18 sets of twins were born per 1,000 births. While the likelihood of having fraternal twins does run in the family,having identical twins does not.

Twins are becoming more common for various reasons. First, more women are becoming mothers at an older age. Older women are more likely to conceive twins because the ovaries release two eggs per ovulation cycle, not one. Older mothers now account for one-third of the twin birth rate, according to the Centers for Disease Control and Prevention.

Reproductive technology is another factor that increases the likelihood of twins. Fertility treatments, specifically in vitro fertilization, often use multiple embryos. When a couple undergoes IVF, a doctor may try to implant two or more fertilized embryos in the hope that one will be viable. If more than one embryo takes, a woman may end up with multiple births. Genetic predisposition also can influence the likelihood of twins, but scientists aren’t sure exactly why.

So, if you continue to hear more about twins these days, don’t be too surprised. As the factors that influence the birth rate of twins continue to rise, so, too, will the number of double-strollers you may see winding along the sidewalks.