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What causes some twin pairs to be born with very different birth weights?

There are several possible explanations for this:

  1. Twin-to-twin transfusion syndrome can cause one twin to be born larger than the other.
  2. Sometimes in a twin pregnancy the placenta does not grow large enough to provide enough oxygen and nutrients to both fetuses. Or one fetus may just have implanted in a location within the womb that gives it better access to nutrition. As a result, one fetus may grow at a slower rate, and would be smaller at birth.
  3. Fraternal (DZ) twins may have inherited different genes from their parents that cause them to grow at different rates in the womb. DZ twins are as genetically similar as non-twin siblings. Non-twin siblings can have very different birth weights. This might explain why some DZ twins have very different birth weights.
  4. Superfetation is the name of an extremely rare situation that has only been reported about 10 times in the medical literature. It occurs when a woman who is already pregnant becomes pregnant a second time with another, younger fetus. This is different from the process of fraternal twinning. In fraternal twinning, the two eggs are produced in the same menstrual cycle. They are fertilized within a few days of each other, and they are implanted in the womb at close to the same time. In superfetation, the second fetus is conceived several weeks after the first fetus, at a time when the first fetus is already developing in the womb. Gestational age is a term that describes how far along a pregnancy is. The two fetuses produced from superfetation would have different gestational ages. They would have different due dates, because they were conceived at different times. At birth, the twin with the later due date would be smaller and less developed than the twin who was conceived first.

Can twins read each other’s minds?

Twins share a special connection beyond that of ordinary siblings. Some people believe that this unique twin bond is endowed with extraordinary supernatural qualities. There are many stories that support this idea.

For example, twins will report that one twin experienced a physical sensation of something that was happening to their twin, such as a heart attack or labor pains. Or that one twin will sense that something is wrong when their twin is in crisis. Sometimes twins will realize that they have performed similar actions when they are apart, such as buying the same item or ordering the same meal in a restaurant. Twins may seem to know each other’s thoughts, by speaking at the same time or completing each other’s sentences.

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As a result of such anecdotal evidence, it has been suggested that twins can read each other’s minds; that they have special twin telepathy; and that they have extrasensory perception (ESP). However, there is no scientific evidence to confirm this.

Despite the lack of scientific proof, these personal experiences do happen to twins. It is generally accepted that they are signs of their deep emotional connection. Also, twins know each other so well that they can often predict how their twin will act or what they will say. This also happens with non-twins who are in a very close relationship, such as spouses who have been married for many years. And many twins, whether due to genes or environment, just have the same instincts, tendencies or preferences, which is why they do similar things, often simultaneously.

While this phenomenon is assumed to be more common in identical twins, it also happens in fraternal twins because they experience a deep emotional connection too.

What are mirror image twins?

Mirror image twins are a type of identical (monozygotic or MZ) twins. The term “mirror image” is used because the twins, when facing each other, appear as matching reflections. They have the same physical features but some are opposite. For example, if one twin is right-handed the other twin may be left-handed. Their hair whorls may curve in opposite directions. Their first teeth may appear in opposite parts of the mouth. Their fingerprints may be mirror images (although identical twins do not have identical fingerprints). In extremely rare cases, one twin may have internal organs on the usual side, while the other twin has them on the opposite side.

All MZ twins develop when one egg is fertilized by one sperm to produce a single zygote. The zygote starts to develop into an embryo, but at some time during the first two weeks after conception, the developing embryo splits into two identical parts. Each part develops into a baby. The two babies are genetically identical: they share 100% of their genes and are always the same sex.

Sometimes, however, the split occurs later than usual in the embryo’s development. Although the developing embryo is still very small when it splits, it already has a right side and a left side. The twins that form in this situation are mirror image twins. About 25% of MZ twins fit this description.

Can twins have different birthdays?

10134707_xxlYes! Twins are defined as children produced in the same pregnancy. Usually they are delivered only a few minutes or hours apart. But they can have different birthdays.

This most commonly happens when labor and delivery begins before midnight on one day and ends after the clock changes to the next day. If that day happens at the end of the month, or even on New Year’s Eve/Day, the two babies can have birthdays in different months and even different years!

Sometimes a twin pregnancy is prolonged to provide each baby with the best chance for survival. If one baby has to be delivered prematurely, doctors can often stop the labor and delay delivery to give the other baby more time in the womb to grow and develop. Twins have been born days and even weeks apart!

Recently, a pair of twins in Ireland set the world record for “longest interval between birth of twins,” at 87 days. For more, click here.

Twins run in my family. Do I have an increased chance of having twins?

There are several factors to consider in answering this question.

Are the twins in the family a result of fertility treatments? These treatments include fertility drugs and assisted reproductive technologies such as in vitro fertilization. The use of such fertility treatments has caused a skyrocketing in the number of fraternal (dizygotic or DZ) twin births (and also higher multiple births). But this does not increase the chances of other family members to have twins.

What kind of twins are in the family? It was believed that identical (monozygotic or MZ) twins occur at random. There is now some evidence to suggest that MZ twinning may run in families, but this is very rare. On the other hand, DZ twins are known to run in some families. DZ twins occur when two eggs are released at the same time and are fertilized by two different sperm. It is believed that there are genetic factors that can cause hyperovulation, which is the release of multiple eggs during ovulation. These genetic factors have not been identified and there is no genetic testing available for them.

On which side of the family are the twins? If DZ twins are on the wife’s side of the family, there is a chance that a genetic factor for hyperovulation runs in the family and that the wife has inherited it. If so, the couple may have an increased chance of having DZ twins. If DZ twins are on the husband’s side of the family, this does not increase the chances that the couple will have DZ twins. However, if the husband inherited a genetic factor for hyperovulation, he could pass it on to his daughter(s). That could increase the chance that the couple will have DZ twin grandchildren!

Factors such as maternal age, race, weight and diet all contribute to twinning. Such factors  may have a stronger influence on twinning than family history. This is discussed in the answer to the question “What factors are related to DZ twinning?

Can identical twins look different?

Yes! Identical twins came from the same sperm and egg, so they have the same chromosomes and genes. But there are environmental differences that can affect the way they look and behave. For example, one twin may have been positioned in the womb in a way that she got more nutrients or blood supply and may be born larger than the other. The twins may have had different childhood illnesses as they grew up. They may have different lifestyles – exercise, smoking, drinking, nutrition, job stress, etc. As identical twins get older they may look more and more different, because they are exposed to more diverse environments.

The science of epigenetics explains how these environmental influences can affect the genes. As a result of the environment, chemicals called “epigenetic marks” attach to the chromosomes and can turn specific genes on or off. So identical twins with identical DNA may have different genes turned on, causing them to look and act differently, and even to develop different diseases such as cancer.

What factors are related to fraternal twinning?

The birth rate of identical (monozygotic or MZ) twins is the same all over the world. It is approximately four per thousand births. But the birth rate of fraternal (dizygotic or DZ) twins varies. DZ twins occur when two eggs are released in the same menstrual cycle and are fertilized by two different sperm. The factors discussed below can affect the process of DZ twinning.

Genetics: There appear to be genetic factors that can cause hyperovulation, which is the release of multiple eggs during ovulation. These genetic factors have not been identified. There is no genetic testing available for them.

Maternal age: The older a woman is, the greater her chance of having DZ twins. This may be due to the higher level of follicle-stimulating hormone (FSH) that older women have as they approach menopause. FSH stimulates ovulation, so an increased level of FSH may cause multiple eggs to be released.  Older women are statistically less likely to get pregnant. But if they do get pregnant, they are more likely to have DZ twins!

Fertility treatments: The use of fertility treatments, such as fertility drugs and in vitro fertilization (IVF), often causes DZ twinning. Fertility drugs given to women to stimulate ovulation can cause the release of more than one egg at a time. DZ twins may occur if two eggs are fertilized.  In IVF, eggs are surgically removed from a woman’s ovaries and combined with sperm in a laboratory. The resulting embryos are transferred into the woman’s womb to develop. To increase the chances of a successful pregnancy, more than one embryo may be transferred into her womb. This may result in the birth of DZ twins.

Race: The occurrence of DZ twins varies by race. DZ twins are more common than average in African Americans populations, and less common in Hispanic and Asian populations.

Nutrition: Being well nourished increases the chance of DZ twinning. The rate of DZ twinning decreases with malnutrition. Some dietary habits, such as eating certain yams grown in Africa, can increase the chances of DZ twinning.

Number of pregnancies: The more pregnancies a woman has had, the greater are her chances of having DZ twins.

Time of year: The most DZ twins are conceived in July. The fewest DZ twins are conceived in January. This may be due to the effect of the length of daylight on the secretion of FSH.

Height: Taller women are more likely to have twins than smaller women. This may be because taller people have more of a substance called insulin-like growth factor (IGF). IGF has been linked to both greater height and increased ovulation.

Why is the number of twins increasing?

The number of twin births has increased dramatically over the past 30 years. A recent government study showed that the twin birth rate rose 76% from 1980-2009! In 1980, one in every 53 babies born in the US was a twin. In 2009, one in every 30 babies was a twin! This increase is due to two factors: the tendency for women to delay having children until they are older, and the increased use of fertility treatments. Both of these factors result in an increase in fraternal (dizygotic or DZ) twins. DZ twins occur when two eggs are released at the same time and are fertilized by two different sperm.

The older a woman is, the higher her chance of having DZ twins. This may be due to the higher level of follicle-stimulating hormone (FSH) that older women have as they approach menopause. FSH stimulates ovulation, and an increased level may cause multiple eggs to be released.  So while older women are statistically less likely to get pregnant, if they do get pregnant, they are more likely to have DZ twins.

Older women are more likely to have trouble conceiving. And fertility problems are common among couples of any age. The use of fertility treatments has soared since 1980, and this has caused a huge  increase in DZ twinning. Fertility treatments include fertility drugs and assisted reproductive technologies.

Fertility drugs given to women to stimulate ovulation can cause hyperovulation (the tendency for multiple eggs to be released at ovulation). This can result in DZ twins.

Assisted reproductive technologies such as in-vitro fertilization (IVF) have greatly increased the rate of DZ twinning. In IVF, eggs are surgically removed from a woman’s ovaries and combined with sperm in a laboratory. The resulting embryos are transferred into the woman’s womb to develop. To increase the chances of a successful pregnancy, multiple embryos are transferred into her womb. This often results in twins and higher multiple births.

The increase in births to older women accounts for only about one-third of the rise in twinning over the last 30 years. The rest of the increase is due to fertility treatments such as fertility drugs and in-vitro fertilization.

What is twin-to-twin transfusion syndrome?

Twin-to-twin transfusion syndrome (TTTS) is a rare condition that occurs only in identical twins when they are in the womb. The cause is unknown. It occurs when blood moves from one twin to the other. The twin that loses the blood is called the donor twin, and the twin that receives the blood is called the recipient twin.

After birth, both babies may have health problems, depending upon how severe the transfusion was. The donor twin is usually born smaller than the other twin, and is usually pale, dehydrated and anemic. The donor twin may have too little blood, and may need a blood transfusion. The recipient twin is usually born larger and may need to have the amount of blood in his/her body reduced. Because of the extra blood, the recipient twin will be red, and will have increased blood pressure. The recipient twin may develop heart problems and may need medications to strengthen the heart.

This condition is usually diagnosed during pregnancy by ultrasound. Treatment before birth may be done. One method type of treatment is frequent amniocentesis around the recipient twin to drain the excess amount of amniotic fluid. Another method of treatment is fetal laser surgery to interrupt the flow of blood from one twin to the other.

Do female identical twins appear more different than male identical twins?

Yes, female identical (MZ) twins can appear more different than male identical twins! The epigenetic factors that can cause identical twins of either sex to look different are discussed in the answer to the question “Can identical twins look different?” In addition, the sex chromosomes are responsible for the greater difference in appearance of some female identical twins.

The sex chromosomes determine a person’s sex. Females have two copies of the X chromosome – one from her father (paternal) and one from her mother (maternal). Males have one X chromosome which came from his mother, and a Y chromosome which came from his father. The Y chromosome has mostly different genes than the X chromosome and is much smaller.

The greater difference in appearance of female MZ twins can be explained by the random inactivation of one X chromosome that occurs in every cell of every female’s body (not just in twins). Remember, females have two X chromosome in each body cell, but males have only one. So that females don’t have a double dose of the products of all the genes on her X chromosomes, one X-chromosome in each female body cell is “turned off” (inactivated).

X-chromosome inactivation occurs very early in development, when the female embryo has fewer than 1000 cells, and it is totally random. From that point on, all cells that develop from each early cell have the same inactive X chromosome – either the maternal X or the paternal X. The genes on the X chromosome that are expressed in each cell are either the maternal or paternal genes.

Since coat color genes in cats are found on the X-chromosome, the effect of random X-chromosome inactivation can be easily seen. The random inactivation of one X chromosome is what causes the coloring in a calico cat, and it is the reason that calico cats are always females, as shown in this video (click to view).

Recently, scientists reported that they have created mice whose X chromosomes from different parents light up as red or green. Click here to read an article about the X chromosome and view photos of the brain, and eyes of these mice. Random patterns of red and green can be seen.

Like calico cats, human females are mosaics of cells in which either the paternal or maternal X chromosome are active. The calico appearance is caused by different versions of just a single gene on the cat’s X chromosome. The human X chromosome has over 1000 genes on it. The maternal and paternal X chromosomes of human females often contain different versions of many of these genes on them.

Random X-chromosome inactivation in MZ twins often occurs after the twins have been created and are developing. It is extremely unlikely that the exact same pattern of inactivation will happen in each developing twin. Each twin will have a different pattern of cells that have either the maternal or the paternal X chromosome active. Each twin will have a different pattern of maternal and paternal genes that can be expressed. This is why random inactivation of the X-chromosome can produce noticeable differences in appearance of MZ female twins.

What is a hidden twin?

In the past, before ultrasound examinations were a common part of prenatal care, parents were sometimes surprised when two babies were born even though only one fetal heartbeat was heard during the pregnancy.

Today, a routine prenatal ultrasound examination can identify a twin pregnancy. If two fetuses are seen by ultrasound, the woman is expecting twins. Although rare, ultrasound pictures can at times be misleading or misinterpreted, especially if they are taken very early in the pregnancy. It is possible that two fetuses could be positioned in such a way that one is directly in front of the other, blocking the second fetus from view. Such “hidden twins” are more likely to occur when the twins are monochorionic (have one chorion, the outer fetal membrane) and positioned closely in their single chorionic sac. However, after 20 weeks of pregnancy, the fetuses have grown enough that a second fetus would be clearly visible on ultrasound, and it would be very unlikely that there is another baby hidden in the womb.

To see a video of how twins interact in the womb, click here!

Can twins have different fathers?

Yes! Fraternal (DZ) twins develop when a woman releases two eggs instead of one during her monthly menstrual cycle. When this occurs and each egg is fertilized by sperm cells from the same man, DZ twins with the same father are created. While rare, it is possible that each egg could be fertilized by a sperm cell from two different men. This could happen as a result of separate acts of sexual intercourse that occur within the woman’s fertile period. DZ twins with different fathers would be created.

The term superfecundation means the fertilization of two (or more) eggs released during the same menstrual cycle by sperm from separate acts of sexual intercourse. A woman’s fertile period can last for five to seven days. So superfecundation that creates DZ twins with the same father may happen often.

But we can only know for sure that superfecundation happened when there is more than one father. When this occurs, it is called heteropaternal superfecundation (hetero = different; paternal = father). Although heteropaternal superfecundation is rare in humans, DNA paternity tests have shown that it does occur.

There are several examples of heteropaternal superfecundation in Greek mythology. For example, the twins Castor and Pollux were said to have different fathers. Pollux was the son of Zeus, and Castor was the son of a mortal man.

Twins with different fathers may also result from fertility treatments in which a mixture of sperm from more than one sperm donor is used.

Can twins be of different races?

Racial differences involve many genes. The genetics of racial appearance is very complex. Below is a simplified answer to this question.

Fraternal (dizygotic or DZ) twins that appear to be of different races have been described. This could happen in several different ways:

  1. Twins that appear to be of different races could result from heteropaternal superfecundation, as described here.
  2. DZ twins, like other siblings, share on average half of their genes. Like other siblings, they can look very similar or very different from each other. In cases where DZ twins are born to parents of different races, one twin could “take after” one parent and the other twin could “take after” the other parent.
  3. Biracial couples can produce DZ twins where one twin looks more like one race and the other twin looks more like the other race. These parents carry genes for physical characteristics from both races. Such genes determine skin color, hair color and texture, eye color and shape, and other physical traits. By chance, the children of mixed race parents could inherit many more of the genes typical of one race than the other race. For example, one twin could inherit more of the genes that determine dark features, and the other could inherit more of the genes that determine light features.

Do identical twins have identical fingerprints?

No! Studies have concluded that, even though the fingerprints of identical (MZ) twins may be very similar, they are not identical. MZ twins have a very high correlation of loops, whorls and ridges. But the details (for example, where skin ridges meet, divide into branches, or end) differ between MZ twins.

MZ  twins share the same genetic makeup (DNA) because they are formed from a single zygote (fertilized egg). However, fingerprints are not an entirely genetic characteristic. They are determined by the interaction of genes in the developmental environment of the womb.

Fingerprint patterns are set between the 13th and 19th weeks of development in the womb. A variety of environmental factors may influence the shape of fingerprints. Examples include differences in umbilical cord length (which can influence blood flow), access to nutrition, blood pressure, rate of finger growth at the end of the first trimester, and position in the womb.

 

Are there more male twins or female twins?

Among non-twin births, males are slightly (about five percent) more common than females. In the US, 105 non-twin males are born for each 100 non-twin females.

However, males are slightly more likely than females to die in the womb. And because the death rate in the womb is higher for twins than for singleton births, female twins are more common than male twins.

Can fraternal twins look so similar that they are mistaken for identical?

 

Ashley Mary-Kate Olsen 2011 Shankbone 2
Yes, it is possible for same-sex fraternal twins to look extremely similar. Like any siblings, fraternal twins are the products of two separately fertilized eggs from the same mother and father. They are genetically as similar as other non-twin siblings. By chance, they could look very similar, or very different, just as non-twin siblings can look very similar or very different. Fraternal twins may look more similar than non-twin siblings because they are the same age, and because they share the same environment (such as prenatal and postnatal nutrition).

 

Does Identical Twinning Run in Families?

Fraternal (dizygotic or DZ) twins sometimes run in families. In the past, researchers believed that identical (monozygotic or MZ) twins do not run in families. They said that MZ twinning was a random event that occurs equally as often in all places and populations around the world. But many people know or have heard of families with more than one set of MZ twins. When several relatives in a family have MZ twin pregnancies, it is called familial MZ twinning. This does indeed occur, although it is very rare.

Familial MZ twinning may actually occur more often than we think. In some cases, familial MZ twinning may be misclassified as familial DZ twinning. This could happen when the twins in the pair don’t look totally identical. And sometimes MZ twin pairs are mistakenly assumed to be DZ twin pairs because they had two chorions at birth instead of one. The chorion is the outermost membrane surrounding the developing embryo. DZ twin pairs always have two chorions. Most MZ twin pairs have one chorion. But one-third of MZ twin pairs have two chorions. This happens when the embryo splits very soon after fertilization. So in a family where several set of twins with two chorions are born, they may be thought to be familial DZ twin pairs when they really are familial MZ twin pairs.

Scientists who study genetics (geneticists) believe that familial MZ twinning might be caused by a mutation (change) in a single gene that controls cell-to-cell connections. The cells of early embryos with this mutation may not stick together as well. This could cause the embryo to split in two before it is implanted in the womb. Some geneticists say that there could be more than one single gene that controls cell-to-cell connections. And they think these genes may be involved at different times in early embryonic development. The evidence for this is that they have only found two types of families with familial MZ twinning. In some families, all the twin pairs had two chorions, because the embryo split very early. In other families, all the twin pairs had one chorion, because the split occurred later. There were no families in which some twin pairs had two chorions and other twin pairs had one chorion. This suggests that mutations in different genes were present in the two different types of families.

Geneticists have analyzed the family trees of all the families with familial MZ twinning studied so far. They say that the gene that causes MZ twinning is inherited in an autosomal dominant pattern. This means that the trait (MZ twinning) occurs in multiple generations, and only one copy of the gene is needed in order for the trait to be expressed.

References:
Shur N. 2009. The genetics of twinning: From splitting eggs to breaking paradigms. Am J Med Genet Part C Semin Med Genet 151C:105–109.

Machin G. 2009. Non-identical monozygotic twins, intermediate twin types, zygosity testing, and the non-random nature of monozygotic twinning: A review. Am J Med Genet Part C Semin Med Genet 151C:110–127.

Machin G. 2009. Familial monozygotic twinning: A report of seven pedigrees. Am J Med Genet Part C Semin Med Genet 151C:152–154.

How often do twin births occur?

In the US, there were 135,336 births in twin deliveries in 2014, the most recent year for which government statistics are available. This represents an increase from 2013, but is lower than the peak years of 2006-09. In Washington State, there were 2,660 births in twin deliveries in 2014.

2014wabirthsbycounty
Percentage of twin births by county, Washington State, 2014

The birth rate for identical (monozygotic, or MZ) twins is the same throughout the world, at about 4 per 1000 births. It does not seem to be related to where people live, what they eat, their racial/ethnic background, or any other factors. This suggests that identical twinning is a random event that is not influenced by genes or environment.

But the birth rates of fraternal (dizygotic or DZ) twins vary by population throughout the world. The highest DZ twinning rate is found among black populations in Africa, and it varies among tribes. The Yoruba tribe in the West African country of Nigeria has an overall twinning rate of 45 per 1000, and 93% of those twins are DZ. This may be related to their diet, which is high in a particular type of yam. This yam contains an estrogen-like substance that may stimulate hyperovulation, which is the tendency to release multiple eggs during ovulation. DZ twins occur when two eggs are released at the same time, and are fertilized by two different sperm.

The DZ twinning rate among African-Americans is higher than among American whites. The lowest rates of DZ twinning are found in Asian populations, where it is about 4 per 1000 births.

DZ twinning seems to run in some families, suggesting that there is a genetic factor causing hyper-ovulation in some women. The racial differences in DZ twinning also suggest the possibility of genetic factors.

The overall rate of twinning in the US has increased dramatically over the last 30 years. In 1980 the rate was 19 per 1000 births. In 2009 the rate had increased to 33.9 per 1000 births. The increase is entirely due to an increase in DZ twin births.

I am a twin. Do I have an increased chance of having twins?

It was once believed that identical (monozygotic or MZ) twins occured at random. There is now some evidence to suggest that MZ twins may run in families, but this is very rare. In general, it is fraternal (dizygotic or DZ) twins that run in families. It is believed that there are genetic factors that can cause hyperovulation, which is the release of multiple eggs during ovulation. DZ twins occur when two eggs are released during the same menstrual cycle, and are fertilized by two different sperm cells. The genetic factors that may cause hyperovulation have not been identified. There is no genetic testing available for them.

Identical twins don't generally run in families, but fraternal twins do.
Identical twins don’t generally run in families, but fraternal twins do.

It looks like twins “skip a generation” in some families. This is because genetic factors that may cause hyperovulation resulting in DZ twins can only be observed in women. But these genetic factors may be inherited by men and passed on to their daughters. Their daughters would then have an increased chance of having DZ twins.

So if you are a female DZ twin and there are other DZ twins in your family, you may have an increased chance of having DZ twins. If you are a male DZ twin and there are other DZ twins in your family, you do not have an increased chance of having DZ twin children. But if you have daughters, you may have an increased chance of having DZ twin grandchildren!

The above discussion does not apply to DZ twins who were conceived by the use of fertility treatments.

Can a male/female twin pair be identical?

 

The term “identical twins” is generally used as a synonym for “monozygotic (MZ) twins.” Both terms assume that that the twins developed from the same fertilized egg (zygote), have identical DNA, and therefore look identical. But, as explained here, identical twins do not always look exactly the same. And male/female twins certainly don’t look identical!

So let’s rephrase the question: Can a male/female twin pair be MZ twins?

The term “MZ twins” simply means that the twins came from the same zygote. Using that definition, the answer is yes! In extremely rare cases, MZ twins that began as a male zygote have developed into a male/female twin pair!

Humans have 23 pairs of chromosome. One of these pairs of chromosomes, called the sex chromosomes (X and Y), determine the sex of the person. Females have two X chromosomes (XX) and males have one X and one Y chromosome (XY). Very rarely, a glitch in copying sex chromosomes can occur, either during the formation of sperm or egg cells, or during the divisions of the zygote after it is formed. The result can be a MZ twin pair of opposite sexes. Below are two possible examples of how this could happen. Remember this is an extremely rare event – only a few cases of MZ twins of opposite sexes have been reported in the medical literature.

First example: After a male (XY) zygote is formed, it starts to develop and split into two embryos. Early in this process, inaccurate copying of the sex chromosomes could result in the loss of the Y chromosome in some cells. The embryo that is formed from the cells missing the Y chromosome would develop into a female (XO). The other embryo would still develop into a male (XY). The twin with only one X chromosome would be female but her cells would have only one copy of the X chromosome. This condition is called Turner Syndrome.

Second example: There could be a glitch in copying sex chromosomes during the formation of sperm or egg cells, resulting in a male zygote that starts out with an extra X chromosome (XXY). Through a complicated series of events, this zygote could result in the birth of MZ twins who are XY (male) and XX (female).

After birth, if one member of a pair of MZ twins had a sex change operation, that would also result in a pair of opposite sex MZ twins.

So, while there is a tiny possibility that a male/female pair of twins could be MZ, it is highly unlikely. It is safe to assume that just about all male/female twin pairs are DZ twins.

References:

Machin G. 2009. Non-identical monozygotic twins, intermediate twin types, zygosity testing, and the non-random nature of monozygotic twinning: A review. Am J Med Genet Part C Semin Med Genet 151C:110–127.

Zech, N. H., Wisser, J., Natalucci, G., Riegel, M., Baumer, A. and Schinzel, A. (2008), Monochorionic-diamniotic twins discordant in gender from a naturally conceived pregnancy through postzygotic sex chromosome loss in a 47,XXY zygote. Prenat. Diagn., 28: 759–763. doi: 10.1002/pd.2031