Are identical twins always the same sex?

Karolina Baker, Alphabiolabs

By Karolina Baker, Health Testing Specialist at AlphaBiolabs
Last reviewed: 17/10/2023

In the UK, 1 in every 60 births are twins, triplets or more, with the number of multiple pregnancies increasing over the last 20 years. Around 11,000 pairs of twins are born in the UK each year. Non-identical, otherwise known as fraternal, twins are the most common and account for two thirds of twins. This means that only one third of twins are identical.

In this article, we explore the science behind identical twin sex determination and how twin DNA (zygosity) testing can help families find out whether twins are identical or non-identical.

What are identical twins? 

Identical twins are a type of twins that originate from the same fertilised egg (zygote). Identical twins are sometimes referred to as monozygotic twins.

Shortly after conception, the developing embryo splits into two separate, but genetically identical, embryos. Once split, the two embryos begin to develop alongside each other to form two identical twin foetuses.

Identical twins are almost genetically identical, with any differences being a result of random mutations that occurred after the embryonic split. However, these differences are generally minimal – meaning that the twins are usually very similar in appearance and can be difficult to tell apart.

As well as random mutations, environmental influences such as prenatal exposure to toxins, nutrition, and lifestyle factors can cause differences in the features of identical twins.

Since they develop from the same fertilised egg (zygote), identical twins will almost always be the same sex. If the original zygote had two X chromosomes, the twins will be female, and if it had an X and a Y chromosome, both twins will be male.

In extremely unusual cases, rare genetic events such as mutation or mosaicism can lead to differences in sex chromosome composition between identical twins. However, the general rule is that identical twins are the same sex.

What are the differences between identical & fraternal (non-identical) twins?

Identical (monozygotic) twins and non-identical (fraternal or dizygotic) twins are two types of twins with several key differences.

The differences lie in the way that the two types of twins originate, their genetic makeup, and how they develop in the womb.

Identical twins

Identical twins originate from a single fertilised egg (zygote) that splits into two embryos during the early stages of pregnancy. As a result, the two genetically identical embryos develop into two babies with nearly identical genetic makeup. Identical twins have the same sex at birth and have the same blood type.

Depending on when the fertilised egg splits, identical twins can share the same placenta and amniotic sac (monochorionic-monoamniotic) or have separate placentas and/or amniotic sacs (monochorionic-diamniotic or dichorionic-diamniotic). Therefore, genetic testing in twins can be very helpful in identifying whether siblings from the same birth are genetically identical or not.

Due to their shared genetic makeup, identical twins normally share a strong physical resemblance to each other.

Fraternal twins

Fraternal twins originate from two separate eggs that were fertilised by two separate sperm cells. Therefore, they share as much DNA as siblings from separate births, which is approximately 50%.

Fraternal twins can be of the same sex or different sexes and may or may not resemble each other closely.

Fraternal twins always have separate placentas and amniotic sacs because they develop from different eggs fertilised by different sperm cells.

In terms of physical resemblance, fraternal twins can differ widely, just like any other siblings.

In summary, the key difference between identical and fraternal twins is the way they develop in the womb and the extent of their genetic similarity. Identical twins originate from a single fertilised egg splitting and are almost genetically identical, while fraternal twins result from the fertilisation of two separate eggs and share around 50% of their DNA.

Are identical twins always the same sex?

Identical twins, also known as monozygotic twins, are almost always the same sex.

This occurs when the fertilised egg (zygote) splits early in development. Typically, the zygote will split so that a female zygote (XX chromosomes) will form two XX female zygotes, and a male zygote (XY chromosomes) will form two XY male zygotes. In other words, identical twins will have the same biological sex.

However, a very rare genetic event can cause the zygote to split into two embryos with different sex-determining chromosomes, resulting in one male and one female identical twin. This rare genetic event usually results in one embryo with Turner syndrome, meaning that the baby will have just one X chromosome as opposed to the usual two, and will exhibit female characteristics. In turn, the other zygote typically has Klinefelter syndrome, with XXY chromosomes, meaning that they inherit an extra X chromosome but will exhibit male characteristics. This rare genetic phenomenon is known as chimerism or mosaicism.

In summary, identical twins are almost always the same sex (both male or female) except in very rare genetic circumstances.

What are semi-identical twins?

As well as identical and fraternal twins, there are other, rarer types of twins. These include semi-identical twins.

Semi-identical, or sesquizygotic twins, occur when a single egg is spontaneously fertilised by two different sperm. This leads to two babies who share all of their DNA inherited from their biological mother, but only around 50% of the DNA inherited from their biological father.

As a result, semi-identical twins can either have different biological sexes, just like fraternal, or non-identical, twins. Semi-identical twins are so rare that they have only ever been seen a couple of times, though experts say there is still much more research to be done on this type of twin.

When can I find out if my twins are identical?

There are several different ways to find out whether your twins are identical or fraternal. The method used primarily depends on the timing of the testing.

Placental examination and ultrasound

During pregnancy, certain medical facts can act as clues as to whether or not your twins are identical. Placental examination by ultrasound can reveal whether your twins share a placenta, and sometimes how many chorions are present.

If your twins share one placenta and one chorion, your twins are likely to be identical. Having two chorions and one placenta means that your twins are more likely to be identical but could be fraternal.

However, an ultrasound may not always provide a clear result. As not all identical twins share a placenta, placental examination is not a definitive method. The only way to truly determine the zygosity status of your twins is by DNA testing.

DNA testing

DNA testing is the most accurate way to determine whether your twins are identical.

DNA testing requires a simple non-invasive buccal (cheek) swab and can be done immediately after birth, or later in life. This test compares the genetic profile of the twins and determines whether they share 100% of their DNA, indicating that they are identical, or 50% of their DNA, meaning that they are fraternal twins.

Can I find out if my twins are identical after birth?

You can find out whether your twins are identical by DNA testing as soon as your twins are born. All that is required is a simple buccal (cheek) swab sample – there is no need to provide a blood sample.

A twin zygosity test can be performed on a person of any age. It’s easy to order an AlphaBiolabs Twin DNA Test online now, for just £99.

Your test kit will contain everything you need to collect the DNA samples at home and return them to our laboratory.

Your secure, password-protected results will be emailed to you the next working day (included in the cost of your test). Alternatively, you can choose to receive your results the same working day, for an extra £70.

Need more information on our Twin DNA Test? Call our Customer Services team on 0333 600 1300 or email info@alphabiolabs.com.

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Karolina Baker

Karolina Baker

Health Testing Specialist at AlphaBiolabs

Karolina joined AlphaBiolabs in 2021, and holds the role of Health Testing Specialist.

As well as overseeing a range of health tests, Karolina plays an active role in the research and development of the company’s latest health test offerings.

Before joining AlphaBiolabs, Karolina worked as an Associate Practitioner at Mid-Cheshire Hospitals NHS Foundation Trust, and as a research assistant at the Turner Laboratory, within the Faculty of Biology, Medicine and Health at The University of Manchester.

Karolina’s main scientific interests include clinical genomics and genetic diagnostics. Her qualifications include a BSc in Molecular Biology and an MSc in Genomic Medicine.

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