Surprise! Pharrell Williams and wife Helen Lasichanh welcomed triplets earlier this month. The now-family-of-six broke the news on Tuesday, with their rep confirming to Vanity Fair that the three new additions are "healthy and happy." While we assume Williams and Lasichanh knew what was coming (with the advanced ultrasound technology we have these days, multiples rarely come as an actual surprise anymore), it's still not every day we hear about someone having triplets. Multiple births are often regarded as a pretty noteworthy event, possibly because their likelihood has declined by 46% between 1998 and 2015 — the most recent year for which the National Vital Statistic Reports analyzed birth data. In fact, 2015 reported the lowest number of triplet and higher-order multiple births (meaning more than two babies at once) in decades. From 2014 to 2015, the rate dropped 9%, going from 113.5 triplets per 100,000 total births to just 103.6. When it comes to something a bit more common, like twins, the rate is 33.5 twins per 1,000 births. All in all, 2015 saw 3,871 triplets, 228 quadruplets, and 24 quintuplets born in the United States. These numbers have more significance when we look at the bigger picture. Between 1980 and 1998, multiple births experienced a 400% boom, but the number has been declining since. The rate of multiple births hasn't been this low since 1992. Triplets can be formed a few different ways. One zygote fertilized by one sperm can split into three, according to American Pregnancy, or one of two zygotes can split, creating two identicals and one fraternal multiple. Often, multiple births happen at random, but if you want to point fingers, there is a clear suspect. IVF, or in vitro fertilization, is pretty widely believed to be behind the original rise of these rates. According to the CDC, IVF is the process of fertilization outside the body. It involves the transfer of one or multiple embryos to the uterus, and these multiple embryos can lead to multiple births. A study in The New England Journal of Medicine reports that, since 1998, doctors have discouraged women from transferring more than two embryos at once for just this reason. Because of this, the rate of multiple births that can be attributed to IVF decreased by 79% between 1998 and 2011. However, IVF is not the only cause. NPR reports that fertility drugs, like clomiphene citrate and injectable hormones, could be to blame. Multiple birth rates due to these drugs rose from 36% to 45% between 1998 and 2011. We don't know whether or not Helen Lasichanh underwent any kind of fertility treatment that could have resulted in triplets, but if not, there's one more thing that could be behind it: age. Women over the age of 30 are more likely to birth multiples, and at 36, Lasichanh falls squarely in this camp. Or, in the words of Pharrell, maybe they just got lucky.