Quadruplet pregnancies, involving four babies developing simultaneously, are notably rare events. Understanding quadruplets involves their statistical infrequency, factors increasing their occurrence, and distinct biological formation processes.
How Rare Are Quadruplets?
Quadruplet pregnancies are significantly less common than single, twin, or triplet pregnancies. Naturally conceived quadruplets are exceptionally rare, with estimates suggesting they occur in approximately 1 in 512,000 to 1 in 729,000 births. In comparison, twins occur in about 1 in 250 pregnancies, and triplets in roughly 1 in 10,000 pregnancies.
In the United States, recent data indicate a very low number of quadruplet births annually. For instance, in 2022, there were only 121 quadruplet births in the U.S. The overall rate of triplet and higher-order multiple births has declined significantly, dropping 62% from 1998 to 2023. This reduction is largely attributed to changes in assisted reproductive technologies (ART) practices.
Factors Increasing the Likelihood
Several factors can increase the chances of conceiving quadruplets, with assisted reproductive technologies (ART) playing a prominent role. Fertility treatments, such as in vitro fertilization (IVF) and the use of fertility drugs, significantly elevate the likelihood of multiple gestations. These methods often involve stimulating the ovaries to produce multiple eggs or transferring multiple embryos, thereby increasing the potential for more than one implantation.
Fertility medications like Clomiphene Citrate and gonadotropins work by stimulating the ovaries. Clomiphene can result in a 5-12% chance of twins and less than 1% risk of triplets or higher, while more potent gonadotropins can increase the twin rate to 30% and triplets or more to 5%. Advanced maternal age also contributes to the likelihood of multiples. Women over 35 are more prone to naturally releasing multiple eggs during ovulation, a natural increase in ovulation before perimenopause.
Genetic predisposition also plays a role, particularly for fraternal multiples. A family history of multiple births, especially on the maternal side, can indicate a genetic tendency for hyperovulation, where more than one egg is released during a menstrual cycle. While identical multiples are generally not hereditary, fraternal multiples can run in families. Other factors such as higher body mass index (BMI) and having had previous pregnancies may also slightly increase the chances.
Understanding Quadruplet Formation
Quadruplets can form through various biological mechanisms, leading to different types: identical (monozygotic), fraternal (dizygotic), or a combination of both. Identical quadruplets result from a single fertilized egg that splits into four separate embryos, each sharing the same genetic material. This means all babies will be the same sex and appear very similar. This type of formation is very rare.
Fraternal quadruplets, the most common type, occur when four separate eggs are fertilized by four distinct sperm. These babies are genetically unique, similar to typical siblings, and may be of different sexes. They develop from independent zygotes.
Mixed quadruplets involve a combination of these processes. For instance, one scenario could be one fertilized egg splitting into identical triplets, with a fourth, separate egg fertilized by another sperm. Another possibility is the formation of a set of identical twins along with two entirely separate embryos, or even two pairs of identical twins.