Twins of different genders are universally accepted as a sound basis for a clinical determination that boy and girl twins are not identical. However, in extremely rare cases, boy and girl twins can stem from the same fertilized egg. 

Twins, who stem from the same fertilized egg, are called monozygotic twins. Washington State Twin Registry stresses that it’s an extremely rare event as only a handful of cases have been reported in the medical literature.

Inaccurate copying of the sex chromosomes in male twins, that began as monozygotic twins, have resulted in a few cases of male/female twin pairs.

A third twin type, called sesquizygotic twins, has been discovered.

Three examples that could result in male/female twins:

• There could be a glitch in copying sex chromosomes, resulting in a male zygote (fertilized egg) that starts out with an extra X chromosome (XXY). This is opposed to the usual XX for a girl or XY for a boy.  Through a complicated series of events, this zygote could result in the birth of monozygotic twins who are male (XY) and female (XX). These twins would be genetically identical except for their sex chromosomes.

• After a male (XY) zygote is formed, it starts to develop and splits into two embryos (identical male twins). 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. Turner Syndrome is a chromosomal condition that affects development in females. These twins would be genetically identical except for their sex chromosomes.

• There is an extremely rare twin type that’s referred to as semi-identical twins or sesquizygotic twins. This occurs when two sperm cells fertilize one egg which then later splits. Semi-identical twins share anywhere from 50 to 100 percent of their DNA. They share 100 percent of their mother’s DNA but only a proportion of their father’s DNA. In 2019 a report was published detailing a newly discovered set of boy-girl semi-identical twins in Australia. They were discovered because they during pregnancy shared a placenta but had different genders (only identical twins can share a placenta, and only fraternal twins can have different genders).

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How are identical twins formed?

A woman ovulates and releases one egg. The egg is fertilized by a sperm cell. The egg splits into two within a few days after fertilization.

The two eggs are developing rapidly into two embryos that have the same chromosomes and genes. This type of twin pregnancy is called monozygotic.

Different kinds of identical twin pregnancies

Identical twin pregnancies are either dichorionic-diamniotic (di-di twins), monochorionic-diamniotic (mo-di) or monochorionic-monoamniotic (mo-mo).

It relates to whether or not the twins share a placenta and sacs (also called membranes). Identical twin pregnancies with separate sacs and placentas are called dichorionic-diamniotic.

Identical twin pregnancies with a shared placenta and a shared outer sac – but separate inner sacs – are called monochorionic-diamniotic.

Monochorionic-monoamniotic is identical twins who share the placenta and both sacs. Mo-mo twin pregnancies are extremely rare.

Are identical twins exactly alike?

Although identical twins share the same genetic characteristics, they are not necessarily exactly alike. Environmental differences in the womb – and when the twins are growing up – can affect the way they look.

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How are semi-identical twins formed?

A woman ovulates and releases one egg. Two sperm cells from two different men enter the same egg and fertilize it. The three sets of chromosomes (as opposed to the usual two) are split into two separate cell sets. 

Some of the cells contain the chromosomes from the first sperm while the remaining cells contain chromosomes from the second sperm, resulting in the twins sharing only a proportion rather than 100 percent of the same DNA from the fathers.

They share the same genetic material from their mother. This type of twin pregnancy is called sesquizygotic.

Do semi-identical twins look alike?

They won’t look as alike as identical twins, but they may have more similarities than some fraternal twin pairs.

How are fraternal twins formed?

A woman ovulates and releases two eggs. The eggs are then fertilized by two different sperm cells (from the same man). This will make the twins share about 50 percent of their DNA.

In very rare cases the eggs can be fertilized by sperm from different men as well. That’s called heteropaternal superfecundation. Genetically that would make the twins half-siblings, and they would share around 25 percent of their DNA.

In both scenarios, the twins would be fraternal twins. That type of twin pregnancy is called dizygotic.

What are fraternal twin pregnancies called?

Fraternal twin pregnancies are always dichorionic-diamniotic (di-di). Fraternal twins have their own separate placentas and sacs.

Do fraternal twins look like each other?

From a genetic perspective, fraternal twins are like ordinary siblings with the exception that they have shared the womb and are born at the same time. “Ordinary” siblings share around 50 percent of their DNA, similar to fraternal twins.

We hope we’ve answered your questions about whether or not identical twins can be male and female. Please leave a reply to this article if you have more questions about monozygotic twins with different genders.

Explains myth

A female fraternal twin has a higher probability of having fraternal twins compared to a woman who’s not a fraternal twin. This is because some women have a hereditary tendency to release more than one egg during ovulation enabling fraternal twins to form. They can pass this tendency on to their daughters. A father who’s a fraternal twin can pass the gene on to his daughters. This may explain why some people mistakenly believe that twins skip a generation.

Increase your chances

There are other factors that predispose you for having fraternal twins. Being tall and overweight seems to increase the chances. Age is also an important factor. Women in their mid- to late thirties are more likely to release more than one egg per menstrual cycle, which means that getting pregnant several times in your late thirties will increase the odds of having fraternal twins. Read more about increasing the likelihood of having twins.

Can twins have twins? We hope we’ve answered your question sufficiently. If you’d like to know more regarding this, then leave a reply below this article and we’ll do our utmost to answer your questions. Also, we would like to get in touch with people, who are twins, and who’ve had twins themselves. Please contact us by writing an e-mail to kate@about-twins.com. 

Identical twins

This type of twin is formed from one fertilized egg being split in two. The eggs develop into two individual embryos that share almost the same genetic material. When developing in the womb, identical twins can share or have their own individual placentas. The same goes with membranes. This sometimes makes it tricky to determine whether or not twin babies are identical or fraternal. Read more about how identical twins are formed to learn how to distinguish identical and fraternal twins from each other, or use our free Twin Test.

Fraternal twins

This type of twin is formed from two eggs being fertilized by two different sperm cells. Fraternal twins are genetically speaking like siblings. They may feel a bit closer, though, as they share a womb and are born at about the same time. As with identical twins there are some patterns of fraternal twinning that are exceedingly rare – read about the different types of twins.

How are twins formed? We hope we’ve answered your questions. Otherwise post a reply below this article and we’ll do our best to assist you. 

One way to tell the difference is using a twin zygosity test, due to the fact that identical twins share almost identical genetic information, while fraternal twins share around half. The twin test is done by swabbing to gently remove cheek (buccal) cells and can be done from home. The procedure is noninvasive and works on twins of any age, newborns to adults. A twin test can be ordered online and is being offered by numerous private companies. However, if you are not up for paying for a DNA-test, you may find your answer by reading the below.

Answer these twin test questions

• Do you have a boy and a girl? Then they are fraternal (but read this).

• Do they have the same sex, AND do you know for sure that they shared a placenta? Then they are identical.

• Do they have the same sex and individual placentas, or is there doubt whether the placentas have fused together? Then they can be either fraternal or identical. In this situation, choosing to be patient may do the trick. According to the Danish Twin Registry you should wait until the newborns are a bit older. If they look so much alike that friends and family can’t tell them apart, they’re likely to be identical. This method works in more than 95 percent of cases, they write on their website.

Di-di – fraternal or identical twins

Identical twins in the womb can have their own separate inner and outer membrane and each have a placenta. They are nicknamed di-di twins (dichorionic diamniotic). Identical di-di twins are sometimes mistaken for being fraternal, as all fraternal twins are di-di twins, whereas this goes for only one third of all identical twins.

Mo-di – identical twins

Identical twins can also be monochorionic diamniotic, named mo-di twins. The twin babies share a single placenta and one single outer membrane, but they each have their own separate inner membrane. This is the case for two thirds of all identical twins making it the most common type of identical twin. Mo-di twins can’t be fraternal. Nevertheless, be aware that placentas can fuse together, fooling health care providers to believe that fraternal di-di twins are in fact identical.

Mo-mo – identical twins

Mo-mo (monochorionic monoamniotic) twins are extremely rare. The twin babies share one single placenta and share both the outer and inner membrane. They only account for about one percent of all identical twins and can never be fraternal.

How are identical twins formed? We hope you now know and that we’ve answered your questions. Otherwise leave a comment below this article. If you’re still not sure whether or not your twin babies are identical or fraternal then consider trying out our Twin Test. 

Consider having (some of ) your children in your thirties

Researchers agree that women in their mid to late thirties are more likely to prepare more than one egg per menstrual cycle than women in their twenties. If two eggs are released the possibility for fraternal twins are present. Getting pregnant several times when you are over the age of 35 will increase your chances of having twins significantly.

Start trying after you stop taking the pill

You need to try conceiving right after you stop taking the pill. This theory based on a 1977 study published in the New England Journal of Medicine suggests that when women first stop taking the pill, their bodies work to re-regulate their hormones. During the first months after getting off the pill, two eggs are sometimes released enabling them to get pregnant with twins. Be aware that other research show, that there is no correlation between ceasing to use oral contraceptives and having twins.

Eat dairy products

Gary Steinman of the Long Island Jewish Medical Center conducted a study that suggests that in particular dairy foods, could boost the production of a protein called insulin-like growth factor (IGF) in women. This could promote the release of eggs by the ovary, so that two are more likely to be released. This is especially the case in areas where growth hormone is given to cattle. Listen to Gary Steinman’s explanation in this video.

Use folic acid as a supplement

Many women take folic acid as a supplement when they’re pregnant. Most do this to reduce the risk of neural tube defects. Some researchers claim that this supplement is also likely to increase the chances of getting pregnant with fraternal twins. This is described in an abstract by Bengt Källén from the University of Lund in Sweden. However, other researchers claim that there is no correlation between the consumption of folic acid and twinning rates. Keep taking the supplement though – it is proven to reduce the risk of neural tube defects.

Be aware of weight

Several studies have showed a correlation between maternal weight and the chances for a twin pregnancy. You have a significantly higher chance of getting pregnant with fraternal twins if you have a BMI over 30. However, obese women are encouraged to lose weight before getting pregnant, as maternal overweight is linked to lower fertility and also increases the risk of babies born with birth defects. The odds of having identical twins do not seem to be related to BMI.

Eat lots of yams

No direct correlation between eating yams and getting pregnant with twins have been proved, but research has suggested that a high occurrence of multiple births could be related to the eating habits of women in specific regions – for instance in a Nigerian town nicknamed Twin Capital of the World – where the intake of yams is known to be high. Read more here.

We hope you’ve learned a bit more about how to have twins. Leave a reply below this article if you have any tips or tricks on how to have twins.