Name for identical twins
Twins - identical and fraternal
Multiple births are more common than they were in the past, due to the advancing average age of mothers and the associated rise in assisted reproductive techniques, in particular the use of fertility drugs. Twins account for over 90 per cent of multiple births. There are two types of twins – identical (monozygotic) and fraternal (dizygotic).
To form identical twins, one fertilised egg (ovum) splits and develops two babies with exactly the same genetic information. This differs from fraternal twins, where two eggs (ova) are fertilised by two sperm and produce two genetically unique children, who are no more alike than individual siblings born at different times. Twins are more or less equally likely to be female or male. Contrary to popular belief, the incidence of twins doesn’t skip generations.
Factors that increase the odds of having twins
Some women are more likely than others to give birth to twins. The factors that increase the odds include:
- Advancing age of the mother – women in their 30s and 40s have higher levels of the sex hormone oestrogen than younger women, which means that their ovaries are stimulated to produce more than one egg at a time.
- Number of previous pregnancies – the greater the number of pregnancies a woman has already had, the higher her odds of conceiving twins.
- Heredity – a woman is more likely to conceive fraternal twins if she is a fraternal twin, has already had fraternal twins, or has siblings who are fraternal twins.
- Race – Black African women have the highest incidence of twins, while Asian women have the lowest.
- Assisted reproductive techniques – many procedures rely on stimulating the ovaries with fertility drugs to produce eggs and, often, several eggs are released per ovulation.
Fertilisation
Hormones secreted by the ovaries, and a small gland in the brain called the pituitary, control the menstrual cycle.
The average cycle is around 28 days. After a menstrual period, rising levels of the hormone oestrogen help to thicken the lining of the womb (the endometrium) and release an egg from one of the ovaries (ovulation).
If the egg is fertilised on its journey down the fallopian tube, it lodges in the thickened womb lining, starts dividing and evolves into an embryo.
Identical or ‘monozygotic’ twins
Around one in three sets of twins is identical. This occurs because the fertilised egg divides in two while it is still a tiny collection of cells. The self-contained halves then develop into two babies, with exactly the same genetic information.
Twins conceived from one egg and one sperm are called identical or ‘monozygotic’ (one-cell) twins. The biological mechanisms that prompt the single fertilised egg to split in two remain a mystery.
Approximately one quarter of identical twins are mirror images of each other, which means the right side of one child matches the left side of their twin.
Fraternal or ‘dizygotic’ twins
Around two in three sets of twins are fraternal. Two separate eggs (ova) are fertilised by two separate sperm, resulting in fraternal or ‘dizygotic’ (two-cell) twins. These babies will be no more alike than siblings born at separate times. The babies can be either the same sex or different sexes, with the odds roughly equal for each.
The proposed ‘third-twin type’
Some researchers believe there may be a third type of twin, although medical opinion is still divided. It is proposed that the egg splits in two, and each half is then fertilised by a different sperm. This theory is an attempt to explain why some fraternal twins look identical.
The normal length of gestation for a single baby is around 40 weeks. However, gestation for twins, either identical or fraternal, is usually around 38 weeks. This shorter time is due to the increased demands on the mother’s body, and the inability of the babies to receive all the nutrients they need in utero.
Since twins are usually premature, they are more likely to have lower birth weights. Prematurity is associated with increased risk of a number of disorders, including jaundice.
Giving birth to twins
Childbirth can give rise to complications when just one baby is present, so twins present extra potential for difficulties. It is advised that women carrying twins give birth in hospital, rather than at home. The babies can be delivered vaginally, but caesarean section delivery may be considered a better alternative in some circumstances.
Zygosity testing
It is difficult to tell if twins are identical or fraternal at birth. Some identical twins may be born with individual sets of membranes, which may lead to the mistaken assumption that the babies are fraternal.
One way to tell the difference is to have the twins DNA-tested. Identical twins share the same genetic information, while fraternal twins share around half.
The test can be done with a sample of cheek cells, collected painlessly. Other tests include blood group examinations.
Where to get help
- Your doctor
- Paediatrician
- Maternity hospital
Things to remember
- Twins account for over 90 per cent of multiple births.
- To form identical or monozygotic twins, one fertilised egg (ovum) splits and develops into two babies with exactly the same genetic information.
- To form fraternal or dizygotic twins, two eggs (ova) are fertilised by two sperm and produce two genetically unique children.
Fraternal twins & identical twins
About twins
We usually describe twins according to:
- how many eggs they develop from – 1 or 2
- whether they share a placenta in their mother’s womb.
The main types of twins are fraternal twins and identical twins.
Most twins are born healthy, but a twin pregnancy can mean a higher chance of health complications for you and your babies.
If you're pregnant with twins, you and your babies will need extra check-ups.
Fraternal twins
All pregnancies start when a sperm fertilises an egg. This fertilised egg is called a zygote.
Sometimes a woman’s ovaries release 2 eggs, and a separate sperm fertilises each egg. This forms twins. These twins are called fraternal twins, dizygotic twins (meaning two zygotes) or non-identical twins.
During pregnancy, the developing babies get oxygen and food from their mother through the placentas and umbilical cords. Fraternal twins have separate placentas and umbilical cords. The technical name for this is dichorionic.
Fraternal twins can be the same or opposite sex and their genes are as different as any other brother and sister. Often, same-sex fraternal twins look different. For example, they might have different hair or eye colour. Occasionally they look quite similar.
Fraternal twins happen in about 70% of twin births in Australia.
Very rarely, fraternal twins share a placenta. These types of twins are called chimaeric twins.
Identical twins
Sometimes a fertilised egg splits within a few days of conception to produce genetically identical twins. Because these twins come from one zygote, they’re also known as monozygotic. Identical twins are the same sex.
There are 3 types of identical twins.
About one-third of identical twins split soon after fertilisation and form completely separate twins. Like fraternal twins, these twins have separate placentas.
The other two-thirds split after they attach to the wall of the womb. As a result, they share a placenta. The technical name for this is monochorionic.
In a very small number of identical twins, splitting might happen even later. In this case, both twins share an inner sac, called the amnion, in addition to sharing a placenta. The technical name for this is monoamniotic twins.
They’re often called MoMo twins.
Identical twins happen in around 1 in 250 pregnancies in Australia.
Although identical twins have the same genes, they don’t always look the same. This is because children’s health and development are shaped not only by genes but also by experiences in the womb and after birth. For example, a twin who gets less blood from a shared placenta might weigh less at birth.
Sharing a placenta or inner sac: why it’s important to know
Sharing a placenta means that twins share a blood supply during pregnancy. Sometimes the blood supply is shared unequally, which can cause health problems for both twins.
Women who are pregnant with twins sharing a placenta need to be checked more often than women who are pregnant with twins with separate placentas. Frequent checks help to pick up any complications early.
Twins sharing an inner sac (monoamniotic) are also at a higher risk of complications during pregnancy because of the chance that their umbilical cords might tangle and cut off their blood supply.
These twins are checked even more closely. Medical professionals often recommend that these twins are born at 32-34 weeks. This is earlier than other types of twins, who are generally born at 37-38 weeks.
Medical professionals use ultrasound to work out how many placentas twins have. The earlier the ultrasound, the more accurately it can say how many placentas there are. It gets harder to work out later in pregnancy. After the birth, medical staff will look at the placentas to check what type of twins they are.
Fraternal or identical: why it’s important to know
Same-sex twins with separate placentas can be fraternal or identical. For health reasons, it’s good to know whether your twins are fraternal or identical.
To find out whether twins are identical or fraternal, you can ask for a genetic test after your babies are born. This is called a zygosity test. The test doesn’t hurt and involves collecting a sample of cheek cells by rubbing the inside of your babies’ cheeks with a soft applicator (like a cotton bud).
The cost of this test starts at $199 (for both twins) in Australia.
Identical twins are more likely than fraternal twins to get the same illness. If one of a pair of identical twins is diagnosed with a particular disease or health condition, like high blood pressure, the other twin should be checked often for early symptoms.
Because of their genetic make-up, identical twins will always be compatible for organ transplantation, if they ever need it. Fraternal twins are compatible only sometimes.
Other reasons why twins and parents of twins might want to know whether they’re fraternal or identical include:
- trying to work out the chance of having more twins in future pregnancies (only fraternal twins run in families)
- making sure they have the right information about their genetic make-up
- being able to answer questions from family, friends and others
- being involved in twin research
- just being curious!
Some identical twins are mirror twins – for example, their hair parts on opposite sides, they are oppositely handed, or they have birthmarks on opposite sides of their body.
In some rare cases, their internal organs can be mirror images of each other. It’s not known why some twins are like this.
In very rare cases, twins can be born physically joined together in different ways. These twins, called conjoined twins (previously called Siamese twins), can happen if the fertilised egg splits quite late after fertilisation.
Triplets (1 in 5000 births) and quadruplets (less than 1 in 100 000 births) can develop as a result of combinations of fraternal and identical twinning. There are no reliable figures for quintuplets (5 babies) and sextuplets (6 babies).
More about twins
In Australia, twins happen in 1 in every 80 births. This means that 1 in 40 Australians is a twin.
The birth rate of identical twins is the same around the world and doesn’t vary with the mother’s age.
In contrast, the birth rate of fraternal twins varies widely across countries and can be influenced by the mother’s age.
Women aged over 35 years are the most likely to have fraternal twins, because their ovaries are more likely to release more than one egg at a time.
More twins were born from the 1990s to the mid-2000s. This is because it was common to transfer more than one embryo during in-vitro fertilisation (IVF) treatments. The rate of twins from IVF is now lower, because it’s now usual practice to transfer only a single embryo.
Rare semi-identical twins born in Australia
Photo copyright, Queensland University of Technology
Image caption,If one egg is fertilized by two sperm cells, a triploid can be formed that has three sets of chromosomes instead of the usual two.
A boy and girl born four years ago in Australia are identical twins on their mother's side. This case is reported in The New England Journal of Medicine.
- Two in one body.
The story of Siamese twins - Medical myths: the ability to roll the tongue into a tube - is it genes?
- In Vietnam, a woman gave birth to twins from different fathers
However, they share only part of their father's DNA, which, from the point of view of genetics, makes them intermediate between monozygotic (identical) and dizygotic (non-identical) twins.
According to experts, this type is extremely rare - usually such embryos do not survive.
Professor Nicholas Fisk, who led a team that looked after the mother and twins at the Royal Women's Hospital Brisbane in Australia in 2014, said the discovery was made during a routine pregnancy scan.
These twins were first discovered before birth. Their mother, who was 28 years old, had not previously had a pregnancy and conceived naturally.
"An ultrasound of the mother after six weeks of pregnancy showed a single placenta and a characteristic arrangement of the amniotic sacs, which indicated the presence of identical twins," says Prof.
Fisk.
"However, an ultrasound performed after 14 weeks of pregnancy revealed that the twins were a boy and a girl, which is impossible with identity."
How could this happen?
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Monozygotic (also called identical or homozygous) identical twins are formed from one zygote (one egg fertilized by one sperm), divided into two or more parts during the crushing stage. They have the same genotypes.
Monozygotic identical twins are always of the same sex and have a very large portrait resemblance. Among monozygotic twins, there is often a great similarity in characters, habits, and even biographies.
Heterozygous (non-identical) twins develop when two eggs are fertilized by two sperm. Heterozygous twins have different genotypes. They are no more similar to each other than brothers and sisters, since they have about 50% of identical genes. Interestingly, in rare cases, heterozygous twins can be born from different fathers. Sometimes heterozygous twins share a fused placenta.
In this case of semi-identical twins, the egg was probably fertilized by two sperm at the same time before it split.
If one egg is fertilized by two sperm cells, a triploid is formed instead of the usual diploid, having three sets of chromosomes instead of the usual two - one from the mother and two from the father.
According to scientists, three sets of chromosomes are incompatible with life and such embryos usually die in the early stages of development.
Semi-identical twins are not named.
Why are twins born?
Image copyright Getty Images
Non-identical twins are more common in families that have had twins before.
They appear more often in late-born mothers, since they often have several eggs at the same time during the process of ovulation.
The appearance of identical twins is not related to whether they were in the family of the spouses. Fertility treatments can increase the chance of having twins.
About 12,000 twins are born in Britain every year.
"Exceptional case"
The first documented case of the birth of semi-identical twins was noted in the USA in 2007.
Professor Fisk notes that an analysis of the statistics available in the world confirms the rarity of this phenomenon.
He and his colleagues examined genetic data from 968 fraternal twins, but could not find any such case.
"This is an exceptionally rare case," says the professor. "While it is useful for physicians to have it in mind when giving birth to identical twins, its rarity means that it is not worth introducing a special genetic test for it.
"
Stories of twins who married twins and became parents of twins
Same wedding dresses and wedding rings, two brides and two grooms who are indistinguishable from each other. This is what double weddings look like for identical twins who marry identical twins. From the outside, this seems like a fairy tale, although psychologists are not surprised by such unions. Mel tells the stories of couples that consist of two sets of twins who marry each other.
Twin City
So you can literally translate the name of the small town of Twinsburg in the US state of Ohio into Russian. The twins Moses and Aaron Wilcox bought a plot of land here in 1818 and began to sell it in parts at low prices in order to attract settlers. Later, they offered to rename the new township from Millsville to Twinsburg, and in exchange allocated a site for a town square and sponsored the opening of a public school.
Twins Moses and Aaron Wilcox Twinsburg is famous for the world's largest, according to the Guinness Book of Records, annual twins festival.
It was held for the first time at 19In 1976, over 77 thousand pairs of twins visited it, including nine from the Soviet Union (of course, they were released only as part of the official delegation).
Universities involved in twin research set up field labs at the festival, and the convention proved to be a great place for twins looking for a mate.
Dean's twin sisters dreamed of marrying twins
Identical twins Brittany and Briana Dean met identical twins Jeremy and Josh Selyers on the last day of the Twinsburg Festival when they bumped into each other in a hotel corridor. In 2019, the twins became celebrities by selling the rights to film a special program about their double wedding to the American broadcaster TLC.
Double wedding of the Celiers brothers and the Dean sisters. Photo: TLC In August 2020, both couples announced that they were expecting their first child at the same time. The girls shared the news on their Instagram page, which is followed by 80 thousand subscribers.
In one of the interviews, they mentioned that they would like to synchronize their pregnancy in order to facilitate the experience of parenthood.
Girls admitted that they always dreamed of marrying twins, although their environment did not take this idea seriously
“People who do not have a twin brother or sister find it difficult to explain the special bond between us. It's like being born with your soul mate and considering it the natural state of things. There were guys who were jealous of my relationship with my sister. I often heard the question: “What are you missing?” I didn’t want to hurt someone’s feelings, but I tried to be honest with myself and felt that I didn’t understand why you don’t have someone as significant as I have ” , Brittany and Briana explain.
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Jeremy and Joshua proposed to their sisters on the same day, but lest they guess, they went for a trick.
They invited the film crew, announcing to the girls that they were supposedly invited to appear in commercials, and during the staged shooting they presented rings. This video was seen on several American TV companies, and as a result, the twins agreed with TLC to film a special issue about their wedding. The girls' wedding rings, by the way, are also the same - in the form of an infinity sign.
Now two families live in the same house: for the twins, who have spent their whole lives next to each other, this is a natural state of affairs, although they admit that they understand why this seems strange to people.
Identical twin parents also had twins
Another pair of twins, Mark and Craig Sanders and Darlene and Diane Nettemeyer, met at the Ohio festival. According to Mark, he fell in love with Diana at first sight, but did not dare to approach the sisters whom he noticed in the crowd: his twin brother was not nearby.
"I was afraid I'd scare them off if I tried to introduce myself like some lonely guy at a twin party, so I needed to find him and show that I'm here for a reason too!" Mark recalls.
In Twinsburg, the girls were visiting relatives, so the brothers had to go to the girls' hometown, Houston, for their first date. It was doubles - the twins went to a baseball game. For a year, the relationship continued mainly at a distance. After the brothers proposed to the girls - also on the same day.
Meeting Day at the Twinsburg Twins Festival, 1998. Photo: Jeff Probst show“We were excited: if one of us proposes, what will the other think when the sister shares the news with her? Will she be invited to marry too? Then we decided that we need to do it at the same time, ”says Craig.
It was easier for the twins to plan the celebration together. For example, in bridal shops, Diana measured one half of the dresses, Darlene - the second: the things that came up first were to the face and the second. Since the girls wanted their father to lead them both to the altar at the same time, they had to abandon puffy skirts in favor of a straighter cut. Otherwise, the three simply would not have had enough space in the narrow church aisle.
Dresses, like wedding rings, were the same.
Both families built their own houses on neighboring plots. They have a common garden where they relax with their five children.
Officially, the children are cousins to each other, but genetically they could be considered relatives, since they were born from two pairs of twins
Craig and Diana are raising twin boys. The birth of twins in the family was not expected: the birth of twins in different generations of the same family is not due to heredity. Identical twins are formed due to the spontaneous separation of a fertilized egg in the womb. So the chance of identical twins having identical twins was one in a million.
The better the twins' spouses know each other, the more harmonious the marriage of both couples will be. between which there is much in common. In addition, twins may be engrossed in a story about themselves, and the “being a twin” attitude becomes part of their identity, without which they cannot imagine themselves.
From the first days of life, twins have a constant companion and often play the role of a significant person for each other - for an only child in the family, for example, it can be a parent. Therefore, it is not easy for them to find a partner: not every person will be ready to put up with a special relationship between twins.
A study conducted in Sweden in 2012 among married twins showed that the more connected the twins' spouses were to each other before the wedding, the more harmonious the marriage of both couples turned out to be.
Scientists also considered marriages to be happy, whose spouses were understanding and accepting of the needs of a spouse who has a twin in maintaining relations with a brother or sister. Rejection of this, in turn, was the second most common reason for divorce among such couples.
The special relationship between the twins, which they do not want to risk by starting to seriously date someone, scientists also considered one of the reasons why twins often tend to start a family later than their peers, the only children in the family.
— Did you fall in love with the same young people?
- No. At dances, they never talked with the same boys. Once my sister was supposed to go on a date, but later she didn't want to. Then I went instead of her. We met with him several times, and in the end he said that it was not at all difficult to distinguish between my sister and me, because we are so different! We had him wrapped around our finger from the very beginning, and he had no idea about it! It was so funny.
Fragment of an interview conducted as part of research
The type of relationship between twins also affects the family life of each of them. Participants in the Swedish experiment admitted that when they found a boyfriend or girlfriend for themselves, it was difficult for their twin to accept it. One of the girls interviewed, for example, said that when she moved in with her boyfriend, her twin sister asked her several times to come home, emphasizing her deteriorating relationship with her mother.
