Multiple Birth

A multiple birth refers to the delivery of two or more offspring from a single pregnancy. While the phenomenon is encountered across many vertebrate species, its frequency varies widely, and in humans it remains relatively uncommon. Multiple births may arise through the division of a single fertilised egg or through the fertilisation of several eggs, generating either genetically identical or genetically distinct siblings. In larger mammals, including humans, high-order multiple births are particularly rare and often associated with increased medical complexity.

Biological Basis and Types of Multiple Pregnancies

Multiple pregnancies occur through several mechanisms. A single zygote may divide to form two or more embryos, producing monozygotic offspring that share identical genetic material. Alternatively, more than one ovum may be released and fertilised, producing dizygotic twins or polyzygotic sets such as triplets or quadruplets, in which siblings share genetic similarity equivalent to that of ordinary full siblings. Splitting can occur at different stages of early development, influencing placental and amniotic configurations.
Monozygotic siblings are always of the same sex due to their identical genetic composition. Dizygotic siblings may be of different sexes and share approximately half of their genetic material. In rare developmental circumstances, sesquizygotic pregnancies occur when a single egg is fertilised by two sperm, giving rise to two embryos sharing an intermediate degree of genetic similarity.

Terminology and Classification

Multiple births are commonly named according to the number of offspring produced:

• Twins: two offspring.
• Triplets: three offspring.
• Quadruplets: four offspring.
• Quintuplets: five offspring.
• Sextuplets: six offspring.
• Septuplets: seven offspring.
• Octuplets: eight offspring.
• Nonuplets: nine offspring.

The classification of multiple pregnancies also includes distinctions based on zygosity and the arrangement of placentas and amniotic sacs. Chorionicity and amnionicity are important medical considerations, as they influence pregnancy risk, fetal development, and clinical management.

Human Multiple Births and Gestation Patterns

In humans, an average singleton pregnancy lasts approximately 38 weeks. Gestation tends to shorten as the number of fetuses increases: twin pregnancies average 36 weeks, triplets around 32 weeks, and quadruplets approximately 30 weeks. Reduced gestational duration elevates the risk of prematurity and contributes to higher rates of neonatal complications.
Advances in neonatal care since the twentieth century have markedly improved survival rates for high-order multiples. Modern trends also show an increase in the incidence of multiple births, largely attributed to assisted reproductive technologies (ART). By the early 2010s, a significant proportion of twins and higher-order multiples in the United States were associated with ART.

Twins

Twins represent the most common form of multiple birth among humans. Statistical data from major health surveillance systems indicate that roughly one in thirty human births results in a twin delivery. Approximately 97 per cent of all multiple births in certain national datasets comprise twins.
Natural twinning occurs in about one in sixty pregnancies, although this rate increases significantly with fertility treatment, reaching up to 20–25 per cent in some settings. Dizygotic twinning may be influenced by maternal genetic traits such as a hyperovulation gene, which increases the likelihood of releasing multiple ova. Fathers cannot influence a partner’s twinning probability directly but may pass this gene to daughters, thereby affecting the subsequent generation. Monozygotic twinning, by contrast, arises randomly and shows no hereditary pattern.

Triplets

Triplet births are substantially rarer than twin births. They may be identical, fraternal, or mixed, depending on whether one or more zygotes divide. The most frequent form involves three separate fertilised ova, though combinations such as one pair of identical fetuses plus a fraternal sibling also occur. Identical triplets are extremely rare and may arise from sequential splitting of a single zygote.
Data from population studies indicate that only a small fraction of all births result in triplets. Among these, identical triplets form a very small minority, estimated at roughly one in ten thousand pregnancies, though precise global figures remain uncertain.

Quadruplets to Sextuplets

Quadruplets occur far less frequently than triplets and historically were exceedingly uncommon until the advent of fertility interventions. Although most quadruplet sets include a mixture of identical and fraternal siblings, a small number of all-identical sets have been recorded. Quadruplets may also be referred to colloquially as quads in British usage.
Quintuplets, or quins, occur naturally at extremely low rates, with historical data suggesting approximately one natural set in many tens of millions of births. The first quintuplets known to thrive beyond infancy were born in Canada in 1934. Modern examples include medically assisted pregnancies documented in media productions.
Sextuplets are exceptionally rare. Notable surviving sets have gained public attention due to the medical complexity and intensive care required. Television series in the United Kingdom and the United States have chronicled the lives of families raising sextuplets, highlighting both clinical and social challenges.

Very High-Order Multiple Births

Instances of septuplets, octuplets, and nonuplets remain globally rare but have occurred in recent decades, often linked to fertility treatments. The first surviving septuplets and octuplets were delivered in the United States in the late twentieth and early twenty-first centuries. In 2021, a record-setting nonuplet birth occurred in Morocco, with all nine children reported as surviving into early childhood. Such events attract substantial scientific and medical interest due to the extremely high risks involved.

Causes, Frequency, and Demographic Trends

The likelihood of multiple births follows observable statistical patterns. Hellin’s law, a traditional approximation, suggests that the frequency of twin births is roughly one in eighty pregnancies, with triplets occurring in approximately one in eighty squared, and so forth. Although actual rates vary and may deviate from this model, the framework remains a useful reference.
Natural dizygotic twinning rates differ between populations and may occur roughly once in every eighty conceptions in parts of North America. Triplets occur at substantially lower natural frequencies, around one in several thousand conceptions.
The rise in multiple births since the late 1970s corresponds with the widespread use of ovulation-stimulating drugs and in vitro fertilisation. These interventions increase the number of mature ova or embryos present in a cycle, thereby elevating the likelihood of polyzygotic pregnancies. Maternal age also influences multiple birth probability; women of advanced reproductive age often have higher levels of follicle-stimulating hormone, potentially increasing the release of multiple ova.