Multiple births vary significantly in their frequency. While twins are a relatively common occurrence, and triplets are known, higher-order multiples like quaternary twins, more commonly known as quadruplets, are considerably rarer. This article explores their definition, prevalence, and contributing factors.
Defining Quaternary Twins
Quaternary twins, or quadruplets, refer to four babies born from a single pregnancy. The formation of quadruplets can occur through several distinct biological mechanisms, leading to different genetic relationships among the siblings.
The most common scenario involves polyzygotic births, where four separate eggs are fertilized by four different sperm. These quadruplets are fraternal and share no more genetic similarity than any other siblings.
Another possibility is monozygotic formation, where a single fertilized egg splits multiple times to produce four genetically identical babies. This type of quadruplet birth is exceedingly rare. Quadruplets can also be a combination of these types, such as two sets of identical twins, or a set of identical twins with two fraternal siblings.
Prevalence of Quaternary Twins
Quadruplets are exceptionally uncommon when compared to single births, twins, or even triplets. Naturally conceived quadruplets are estimated to occur in approximately 1 in 700,000 to 1 in 729,000 pregnancies. Some estimations place the odds even lower, at about 1 in 571,787 pregnancies for spontaneous multiples of any zygosity. In 2022, for example, there were only 121 quadruplet births in the United States, a stark contrast to over 100,000 twin births in the same year.
The incidence of higher-order multiple births, including quadruplets, has fluctuated over time, particularly influenced by medical advancements. While the rate of triplet and higher-order multiple births saw a significant increase between 1980 and 1998, it has since declined. From 1998 to 2023, the rate of triplet, quadruplet, and higher-order multiple births in the U.S. dropped by 62 percent. This reduction brings the 2023 rate of 73.8 per 100,000 total births closer to levels seen in the early 1990s.
Factors Influencing Quaternary Twin Births
The primary driver behind the occurrence of quadruplet births today is the use of assisted reproductive technologies (ART), such as in-vitro fertilization (IVF) and ovulation-stimulating medications. These treatments work by increasing the number of eggs available for fertilization or by transferring multiple embryos into the uterus, significantly raising the likelihood of multiple pregnancies. It is estimated that approximately 90% of quadruplets are conceived with the assistance of medical technology.
Initially, ART practices often involved transferring multiple embryos to maximize the chances of a successful pregnancy, which led to a rise in higher-order multiple births. However, medical guidelines and practices have evolved to mitigate the risks associated with multiple pregnancies for both mother and babies. Current recommendations often prioritize single embryo transfer (SET), aiming to reduce the multiple birth rate while maintaining high live birth rates. This shift in practice has contributed to the recent decline in triplet and higher-order multiple births, even as ART use remains prevalent.