How does twins form

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.

Birth of twins: what is the probability? | PHARMACY

Twins or dizygotic twins are born when 2 different eggs are fertilized by 2 different sperm at the same time. Identical twins, or homozygous twins, give birth if 1 egg is fertilized by 1 sperm and then divides, forming 2 embryos. Identical twin births occur around the world with approximately the same frequency of 3-4 cases per 1000 births, the frequency of birth of dizygotic twins varies significantly around the world, from 6 cases per 1000 births in Asia to 40 cases per 1000 births in Africa. nine0003

Scientists believe that the birth of twins is based on a genetic component, but the details of this relationship have not previously been sufficiently studied. In a new study, scientists from the Free University of Amsterdam (Vrije Universiteit), the Netherlands, have identified 2 gene variants associated with the birth of dizygotic twins.

The results of the study are relevant for the European population, since the population of Asia and Africa may have additional gene variants, scientists note. nine0003

Until now, researchers have not been able to analyze in detail some of the factors that likely increase a woman's chances of having twins. For example, women who became mothers in adulthood and those who had relatives who gave birth to twins were more likely to have dizygotic twins than other females. Therefore, the researchers decided to analyze the genes responsible for the birth of twins. They started by collecting genetic information from databases in the Netherlands, Australia, and the United States. As a result, they identified 1980 mothers of twins conceived without infertility treatment and compared their genome with that of 12,953 women from the control group who did not give birth to dizygotic twins.

After searching for and isolating the genetic variants that distinguish mothers from the two groups, the scientists sent the results to their colleagues in Iceland, who conducted a similar study on 3,597 mothers who gave birth to dizygotic twins and 297,348 women from the control group.

This allowed us to identify 2 gene variants that appeared more often in mothers of twins than in mothers who gave birth to 1 child. One gene variant is associated with increased levels of follicle-stimulating hormone (FSH) in women, and also plays an important role in reproductive function in men. FSH stimulates a woman's ovaries to release an egg. Its elevated level can lead to the simultaneous release of several eggs from the ovaries, which can subsequently lead to the fertilization of 2 eggs at once and the birth of twins. nine0003

The second variant of the SMAD3 gene is involved in cell signaling and, according to the researchers, likely plays a role in how a woman's body responds to FSH. If a woman's ovaries are sensitive to FSH, this can lead to the release of several eggs at once and their subsequent fertilization, even if her body produces FSH in moderate amounts.

Interestingly, the presence of the first gene variant in a woman increases the relative probability of having twins by 18%, and the second - by 9%. The combination of these gene variants increases the chances of having twins by 29%.

Scientists note that these 2 gene variants do not show the full picture of the influence of the genome on the birth of twins, perhaps in the future additional genes will be identified that play a role in this process.

Based on www.medicaldaily.com

How twins or twins appear

What can we advise you? nine0026

Twins (or dizygotic twins) are born when 2 different eggs are fertilized by 2 different sperm at the same time.

Identical twins (or homozygous) are born when 1 egg is fertilized by 1 sperm and then divides to form 2 embryos.

Identical twins are born around the world with approximately the same frequency of 3-4 for every 1000 births. The birth rate of dizygotic twins varies considerably around the world, from 6 per 1000 births in Asia to 40 per 1000 births in Africa. nine0028

Identical twins have the same set of genes. Using conventional DNA analysis, it is almost impossible to distinguish between their biological samples (blood, saliva, hair).

Scientists believe that the birth of twins is based on a genetic component, but the details of this relationship have not been sufficiently studied before. In a new study, scientists from the Free University of Amsterdam (Vrije Universiteit), the Netherlands, have identified 2 variants of a gene associated with the birth of dizygotic twins. nine0028

One gene variant is associated with increased levels of follicle-stimulating hormone (FSH) in women. FSH stimulates a woman's ovaries to release an egg. Its elevated level can lead to the simultaneous release of several eggs from the ovaries, which subsequently can lead to the fertilization of 2 eggs at once and the birth of twins.

The second variant of the SMAD3 gene is involved in cell signaling and, according to the researchers, probably plays a role in how a woman's body responds to FSH. If a woman's ovaries are sensitive to FSH, this can lead to the release of several eggs at once and their subsequent fertilization, even if her body produces FSH in a moderate amount. nine0028

Interestingly, the presence of the first gene variant in a woman increases the relative probability of having twins by 18%, and the second - by 9%. The combination of these gene variants increases the chances of having twins by 29%.