History of Twin in Timeline

Twins are offspring from the same pregnancy, classified as either monozygotic (identical) or dizygotic (fraternal). Identical twins originate from a single zygote that splits, resulting in shared genetic material and always the same sex. Fraternal twins develop from separate eggs fertilized by different sperm, leading to potentially different sexes and varying genetic similarity. While identical twins share the same parents, rare cases of fraternal twins may have different fathers despite sharing the same mother.

1980: Rise in twin births begins in the US

From 1980 to 1997, the number of twin births in the United States rose 52%, partly attributed to the increasing use of fertility drugs and procedures like IVF.

1980: Twin birth rate increase in the US

In 1980, the human twin birth rate in the United States was 9.4 twin sets (18.8 twins) per 1,000 births.

1981: Study on a deceased XXX twin fetus

In 1981, a study of a deceased XXX twin fetus without a heart suggested it was likely a polar body twin, though the authors couldn't predict if a healthy fetus could result from polar body twinning.

1992: Study estimates rate of heteropaternal superfecundation

A 1992 study estimated that the frequency of heteropaternal superfecundation among dizygotic twins, whose parents were involved in paternity suits, was approximately 2.4%.

1993: Mixed twins born to biracial couple

In 1993, mixed twins, exhibiting differing ethnic and racial features, were born in London to a white mother and Caribbean father.

1997: Twin births continue to rise

From 1980 to 1997, the number of twin births in the United States rose 52%. This rise can at least partly be attributed to the increasing popularity of fertility drugs and procedures such as IVF, which result in multiple births more frequently than unassisted fertilizations do.

2003: Study on triploidy and sesquizygotic twinning

In 2003, a study argued that many cases of triploidy arise from sesquizygotic (semi-identical) twinning, which occurs when a single egg is fertilized by two sperm and splits the three sets of chromosomes.

2006: Dairy and dizygotic twinning study

A 2006 study found that insulin-like growth factor in dairy products may increase the chance of dizygotic twinning, with vegan mothers having a lower likelihood of twins compared to vegetarian or omnivore mothers.

2007: Study of living twins with different paternal chromosomes

In 2007, a study reported a case of living twins sharing identical maternal chromosomes but distinct paternal chromosomes from the same man. Both twins were chimeras, one intersex XX and one XY male, with an undetermined fertilization mechanism.

2007: Theory on monozygotic twinning related to IVF

In 2007, a theory related to in vitro fertilization (IVF) suggests that monozygotic twins may form when a blastocyst contains two inner cell masses, each leading to a separate fetus, instead of the embryo splitting during hatching.

2008: Caesarean section rate for twin pregnancies in the US

In 2008, it was estimated that 75% of twin pregnancies in the United States were delivered by caesarean section.

2009: Twin birth rate peaks in the US

By 2009, the human twin birth rate in the United States rose to 16.7 twin sets (33.3 twins) per 1,000 births, marking a 76% increase from 1980.

2011: Twin births and assisted reproductive technology

In the United States, by 2011 it was estimated that 36% of twin births resulted from conception by assisted reproductive technology.

2012: Study finds healthy fetus can result from a polar body

In 2012, a study found that it is possible for a polar body to result in a healthy fetus, contradicting earlier uncertainties from 1981.

2014: Twinning and human care

In 2014, Ward touched on the higher twinning propensity in species that are under human care, which is thought to be caused by random mutation facilitated by genetic drift.

2016: Identical twin puppies discovered

In 2016, a C-section of an Irish Wolfhound revealed identical twin puppies sharing a singular placenta. South African scientists confirmed monozygotic twinning in the dog using DNA profiling.

November 17, 2018: Twins born 97 days apart

On November 17, 2018, the first of twins was born in Cologne, Germany, with the second twin born 97 days later, possibly setting a worldwide record for the duration between twin births.

2018: Marco Varella's study on dizygotic twinning

In 2018, Marco Varella studied the origin of dizygotic twinning. Garbino and Varella proved that smaller species experiencing infertility in old age or unstable habits due to predation or human interference have undergone natural selection, gaining higher twinning propensities.

2018: Varella's research on cebidae twinning

In 2018, Varella observed that monkeys in the cebidae genus are more likely to produce twins because of their small size and insect-based diet. Smaller size indicates shorter gestation and rapid maturation, leading to shorter lifespans. Cebidae's insectivorous diet correlates with their heightened ability to reproduce. Older mothers within the cebidae genus have a higher chance of producing twins. The cebidae genus has a high mortality rate attributed to their size, meaning that in order to ensure generational survival, they produce an excess of offspring in ensuring generational survival.

2019: Limited cases of semi-identical twins

As of 2019, only two cases of semi-identical twins, also known as sesquizygotic twins, have been reported worldwide. These twins are hypothesized to occur when an ovum is fertilized by two sperm.

2019: Genetic basis of semi-identical twins reported

In 2019, Michael Gabbett and Nicholas Fisk reported the likely genetic basis of semi-identical twins, documenting a second case of sesquizygosis and presenting molecular evidence that two sperm from the same man fertilized an ovum simultaneously.

January 2021: Research suggests identical twins have genetic differences

In January 2021, research from Iceland published in Nature Genetics suggested that identical twins have genetic differences originating in early embryonic development, challenging the previous assumption of complete genetic identity.

2021: Guilherme Siniciato Terra Garbino's study on bat litter size

In 2021, Guilherme Siniciato Terra Garbino's study on bat litter size revealed that the vespertilionidae genus has higher twinning propensities due to their roosting habitats. The common ancestor of bats had a higher twinning propensity which was lost and regained eighteen times in evolutionary history. While subfamilies like Myotinae and Murinae lost the twinning trait, Vespertilionidae retained it due to mutation and environmental conditions triggering natural selection. Vespertilionidae's roosting locations increased mortality, but natural selection favored high twinning propensity, enhancing their reproductive survival.

2021: Non-human dizygotic twinning

In 2021, it was noted that non-human dizygotic twinning is a common phenomenon in multiple animal species, including cats, dogs, cattle, bats, chimpanzees, and deer.

2022: High twinning propensity positively correlated with infant mortality rate

In 2022, Rickard stated that the high twinning propensity in species is thought to be positively correlated with the infant mortality rate of the reproducing organism's environment.