Seeing double? Here's the difference between identical and fraternal twins

Nikki Stevenson
Nikki Stevenson
Nikki is a parenting writer and a mom to three wild boys who keep her on her toes (and occasionally make her question her sanity). With over 15 years of experience in the parenting industry, she has more tips and tricks than Mary Poppins on speed dial. When she's not typing away at her keyboard, you can find her sipping on coffee, hiding in the bathroom for five minutes of...
Created on May 17, 2024 · 15 mins read

The journey of pregnancy can sometimes double the surprise—literally! For those expecting twins, you get taken down a pretty interesting side route.


From how they form to the way they interact with the world, each type of twinship offers a glimpse into the remarkable processes of human development. If you’ve ever wondered why some twins look exactly alike and others don’t, or how twin pregnancies occur, this guide is for you.

We’re going to dive into the science behind the zygotic miracles that result in twins, from the splitting of a single fertilised egg to the simultaneous fertilisation of two separate eggs; uncovering the fascinating genetic, environmental, and biological factors that influence twinning.

Whether it’s the genetics of identical twins or the slightly varied DNA of fraternal twins, the journey into their creation is as captivating as the twins themselves.

Identical vs fraternal twins


When it comes to twins, the terms ‘identical’ and ‘fraternal’ are more than just labels—they define how the twins are conceived and how closely they will share genetic traits.

  • Identical twins (or monozygotic twins): originate from a single fertilised egg that splits into two. This split results in two individuals who share the same genetic makeup, making them virtually mirror images of each other with very similar physical traits and, in very rare cases, almost indistinguishable DNA profiles.
  • Fraternal twins (or dizygotic twins): develop from two separate eggs, each fertilised by its own sperm. Fraternal twins share about 50% of their DNA, just like any other siblings, but they happen to share the same womb even though they come from more than one egg. Consequently, fraternal twins can be of the same or different sexes and may not necessarily resemble each other more than any brothers or sisters.

This fundamental difference highlights the unique nature of twin relationships and twin pregnancies. Whether identical or fraternal, twins have a special bond, but understanding their type helps clarify many aspects of their future health, development, and individual identities.


Do identical twins have the same blood type?


Ever wondered if identical twins also share the same blood type? The answer generally leans towards yes. Since identical twins, or monozygotic twins, come from a single fertilised egg that splits into two, they typically share the same genetic makeup, including their blood type. T

This genetic mirroring results from both twins inheriting the same combination of alleles from their parents, which determines their blood characteristics.

However, in very rare cases, differences can arise due to mutations or changes during the development stages. But for the most part, identical twins sharing the same blood type is just another facet of their profound genetic similarity that stretches beyond mere appearances to more intrinsic biological traits like blood type and even potential medical predispositions.

Understanding these genetic intricacies can be crucial, especially in medical scenarios where blood transfusions or organ transplants are involved, as twins can sometimes serve as perfect matches for each other, further showcasing the unique biological bond they share.


What are your chances of having identical or fraternal twins?


It’s no secret that many of us wonder about the likelihood of welcoming two bundles of joy at once. The chances of having a twin pregnancy, whether identical or fraternal, can vary significantly based on several factors.

Identical twins occur in about 3 to 4 per 1,000 births worldwide, which remains fairly constant across populations. This type of twinning happens when a single egg is fertilised and then mysteriously splits into two embryos, leading to siblings with exactly the same genetic blueprint.

On the other hand, a fraternal twin, also known as dizygotic twins, results from two separate eggs being fertilised by two separate sperm in a twin pregnancy. The rates of fraternal twinning can vary greatly and are influenced by factors such as genetics, the mother’s age, and the use of fertility treatments.

For instance, fraternal twinning is more common in certain ethnicities like Africans, and less common among Asians and Hispanics. Women over the age of 35 and those using fertility treatments see higher rates of fraternal twins due to increased ovulation of multiple eggs.

Interestingly, family history plays a role too. If you have fraternal twins in your family, your chances of having twins increase. However, this genetic predisposition applies only to the mother’s side. If she carries genes for hyperovulation, she’s more likely to release multiple eggs during ovulation, increasing her chances of conceiving fraternal twins.

What are the odds of fraternal twins being the same sex?


You might be curious about whether your adorable fraternal twins might be boys, girls, or one of each?

It’s like rolling dice in a game of nature’s odds. Statistically, fraternal twins, resulting from the fertilisation of two separate eggs by two separate sperm, can either be the same sex or a combination of both. Each baby develops independently with their own genetic profiles.

Here’s the scoop: there’s a 50% chance that fraternal twins will be the same sex—either two boys or two girls. The other 50% of the time, the twins will be boy-girl.

This is because each egg and each sperm carry a sex chromosome independently, leading to the possible combinations that form either male (XY) or female (XX) twins.

These odds make every pregnancy with fraternal twins a unique journey into the mysteries of genetics. Unlike identical twins who are always the same sex due to sharing the same DNA, fraternal twins can mix and match, adding a delightful element of surprise to your twin adventure.

Whether they end up being brothers, sisters, or one of each, fraternal twins often share a deep bond, shaped by their shared prenatal environment and early life experiences together.

Can non-identical (fraternal) twins have different dads?


Imagine this scenario: you’re having a casual chat at the playground, and the topic of fraternal twins comes up. Someone quips, “Did you know fraternal twins can have different fathers?” and suddenly, you’re all ears.

It sounds like a storyline lifted from a daytime soap opera, but it’s a genuine biological phenomenon called superfecundation.

In very rare cases, fraternal twins can indeed have different fathers. This happens when two eggs released during the same menstrual cycle are fertilised by sperm from separate instances of sexual intercourse, involving different men.

The medical community refers to this as heteropaternal superfecundation. It’s a rare event, but it has been documented and adds an intriguing layer of complexity to the biology of reproduction.

While the odds are slim, the possibility exists due to the fertility window during which a woman can conceive. If multiple eggs are present, and there’s more than one sexual partner involved close in time, each egg can potentially be fertilised by sperm from different fathers. These scenarios are more likely to come to light thanks to modern DNA testing, leading to some astonishing family discoveries.

So next time you hear someone musing about twins with different dads, you’ll know that while it’s not common, it’s entirely within the realm of possibility. It’s one of those remarkable examples of just how surprising and diverse human biology can be!

Twins who are of different gestational ages


Superfetation is extremely rare but fascinating. It occurs when a second, new pregnancy happens during an ongoing pregnancy.

A woman releases an egg that gets fertilised while she is already pregnant, leading to twins with different gestational ages. This can result in the babies being born days or even weeks apart, each reflecting their own unique developmental timeline.

This rarity is so because most women’s bodies naturally prevent ovulation during pregnancy through hormonal changes that maintain the uterine environment for the growing fetus. However, in very rare cases, these mechanisms might not fully work, allowing for a second ovulation and subsequent pregnancy.

Imagine the surprise and unique challenges that come with managing a pregnancy where the twins are not only at different developmental stages but might even have different health needs. It’s a scenario that requires meticulous medical attention and has fascinated doctors and parents alike.

Female and male identical twins


The term ‘identical twins’ often brings to mind images of two people who look strikingly similar, sharing the same eye colour, hair, and infectious laugh. But can identical twins be of different sexes? Here’s a bit of genetic intrigue that often sparks curiosity at family gatherings.

Typically, identical twins (known scientifically as monozygotic twins) are formed when a single fertilized egg splits into two, leading them to share identical DNA. This usually means they will be the same sex because they have the same chromosomal makeup. However, in exceedingly rare cases, genetic variations or anomalies can occur.

One such scenario involves Turner Syndrome, where one twin might lose part of a sex chromosome and, thus, the twins can appear to be different sexes. Another instance might involve genetic mutations or discrepancies in how sex hormones are processed by the body. Although these cases are medical rarities, they give quite an insight into the complexities of genetics and embryological development.

Semi-identical twins (polar body twins)


Now, let’s get into one of nature’s more unusual creations: semi-identical, or polar body twins.

Because amidst all this talk of identical versus fraternal, it’s possible that your twins aren’t entirely identical nor completely fraternal, but somewhere beautifully in between!

Semi-identical twins occur in a very rare twinning process where one egg is fertilised by two separate sperm. This unique situation leads to twins who share exactly half their DNA from their mother but have different sets of DNA from their father. It’s a fascinating genetic mix that places polar body twins in a category all their own.

This form of twinning is so rare that many parents might never even realise this could be a possibility until genetic testing reveals the remarkable nature of their children’s DNA. These twins can be more similar than typical fraternal twins yet not as identical as monozygotic twins.

Why should you find out if your twins are identical or fraternal?


As a mum, knowing whether your little duo is identical or fraternal isn’t just about satisfying your curiosity—it’s about being armed with essential knowledge that could influence their health and development.

The medical benefits

Identifying whether your twins are monozygotic (identical) or dizygotic (fraternal) can provide crucial insights into the medical care they may need. For instance, identical twins share the same genetic blueprint and are more likely to develop the same genetic disorders.

This shared genetic identity also means they can usually accept organ transplants from each other without rejection, an invaluable advantage should the need ever arise.

Supporting their individuality

Moreover, knowing the type of twins you have affects how you approach their individuality. Identical twins might look the same and share many preferences, but they are distinct individuals with unique needs and personalities.

With this in mind, you can nurture their individual traits from an early age and foster a healthy sense of self despite their shared genetics.

On the other hand, fraternal twins can be as different as any other siblings who happen to share the same birthday. They can have different fathers, a phenomenon known as heteropaternal superfecundation, and they might even exhibit significant differences in interests and aptitudes.

So, while the allure of twin mysteries is captivating, the real reward lies in how this knowledge helps you raise happy, healthy, and well-understood children.

How twins actually benefit scientific research


The world of twins provides families and scientists with a fascinating window into the nature of human development and genetics. Twin research, particularly studies involving identical twins, has been pivotal in unraveling the complex interplay between genetics and environment in shaping us.

Studying identical twins

When scientists study identical twins, who share the same DNA, they gain insights into how different environments can lead to different outcomes in individuals with the same genetic makeup.

This research helps us understand various health conditions and traits, ranging from susceptibility to diseases to personality development.

It also sheds light on the effects of shared environments compared to unique experiences that each twin might have.

Studying fraternal twins

Fraternal twins are equally important in scientific studies.

Since they share about 50% of their DNA, similar to typical siblings but are born at the same time, they provide a different data point. Comparing and contrasting fraternal twins allows researchers to further dissect the roles of shared prenatal environments and genetic factors.

Research involving twins has led to breakthroughs in understanding complex disorders such as diabetes, heart disease, and mental health issues. Twin studies often help identify genetic markers and risk factors, offering potential pathways for early interventions and tailored treatment plans.

Wrapping up the different twin types


When it comes to understanding twins, the diversity extends beyond the commonly known identical and fraternal categories. Here’s a concise guide to the various types of twins you might encounter:

1. Identical twins (monozygotic twins)

Result from a single fertilised egg splitting into two, making identical twins happen. These twins share the same genetic makeup and are always of the same sex. They can share one placenta or have separate placentas depending on when the egg splits.

2. Fraternal twins (dizygotic twins)

Develop from two eggs, each fertilised by its own sperm. Fraternal twins can be of the same or different sexes and each has its own placenta. Genetically, they are no more similar than any other siblings.

3. Conjoined twins

A rare occurrence where twins are physically connected to each other at some part of their bodies, referred to as conjoined twins. This happens when the division of the fertilised egg occurs later than usual, typically more than 12 days after fertilisation. Conjoined twins may share organs, tissues, or simply be connected at a certain point on their bodies, making separation a complex and delicate process. Each case of conjoined twins is unique, with survival and surgical possibilities varying greatly.

4. Semi-identical twins (polar body twins)

These twins are the result of one egg being fertilised by two sperm before splitting. They share exact DNA from their mother but different DNA from their father, placing them somewhere between identical and fraternal.

5. Mirror twins

A subset of identical twins where the twins develop asymmetrically, mirroring each other. For instance, one might be left-handed while the other is right-handed.

6. Half-identical twins

These occur when the egg splits before fertilisation, and each half is then fertilised by a different sperm. This type results in twins sharing 50% of their mother’s DNA but different DNA from their father.

Twin diversity and factors influencing twinning


Multiple births

Refers to pregnancies where more than one fetus is carried to term. These can include combinations of different types of twins and higher-order multiples like triplets and quadruplets.

Twin pregnancy

A pregnancy involving either identical or fraternal twins, each with its own characteristics and medical considerations.

Egg splits and fertilisation

The process of twinning can vary widely, from a single egg splitting post-fertilisation to multiple eggs being fertilised during a single cycle.

Genetic and environmental influences

Various factors can increase the likelihood of having twins, including family history, maternal age, and the use of fertility treatments.

Things to bear in mind for your twin pregnancy


For parents carrying a dynamic duo, your pregnancy and labour will (as you can imagine) look pretty different from those with a one-hit-wonder. Whilst the normal pregnancy length for one baby is 40 weeks, twins can push this a bit earlier to 38 weeks or less. Pregnancies with multiple babies have a 68% chance of premature birth and being placed in a Neonatal Intensive Care Unit as the birth is considered a higher risk.

Multiple birth pregnancies can carry extra complications like:

  • Premature birth: When birth happens before 37 weeks of pregnancy.
  • Preeclampsia: A disorder of pregnancy involving high maternal blood pressure and protein in the urine. This affects about 4-5% of pregnancies in Australia, but only 1-2% of cases are severe enough to be life-threatening to mum and bub.
  • Preterm labour: Refers to labour and contractions happening before 37 week s of pregnancy.
  • Hyperemesis gravidarum: Severe nausea and vomiting in pregnancy that lasts more than a couple days.
  • Gestational diabetes: Diabetes that is diagnosed for the first time during pregnancy, after high blood sugar levels.
  • Anemia: Your blood has a lower-than-normal amount of red blood cells which carry oxygen around the body.
  • Cholestasis of pregnancy: A liver condition that can occur later in pregnancy, with main symptom being severe itching and yellowness in the skin.
  • Polyhydramnios: The buildup of accumulated amniotic fluid, which can lead to further labour complications.
  • Bleeding: Spotting can be more common in multiple birth pregnancies. If this is accompanied by intense cramping or heavy bleeding, head to the hospital.

Whilst these complications are certainly not guaranteed, and many mums have a straightforward pregnancy and birth with multiple babies, your GP or healthcare provider will keep all of this in mind while supporting your journey

Wrapping it up


By learning whether your twins are identical or fraternal (and understanding the distinction), you can practically benefit their health, your approach to parenting, and even their individual identities.

It helps anticipate and address specific medical and developmental needs that may arise. It also enriches your understanding of their unique bond and individual differences.

An identical twin might share an incredible closeness with their other half due to their shared genetics, but they also face unique challenges such as being seen as ‘the twins’ rather than as individuals. Fraternal twins, while not as genetically close, might exhibit a wider range of differences in interests and personalities, which can shape how they interact with the world and each other.

Ultimately, whether navigating health concerns or fostering individual growth, understanding the nuances of twin types equips you to better support and celebrate your children as distinct individuals with a shared beginning. So, as you continue on your twin journey, cherish the joys and challenges alike, knowing that each day with your twins is a new page in an extraordinary story.

Sources


Cutler, T.L. et al. (2015) ‘Why accurate knowledge of Zygosity is important to twins’, Twin Research and Human Genetics, 18(3), pp. 298–305. doi:10.1017/thg.2015.15.

Smits, J. and Monden, C. (2011) ‘Twinning across the developing world’, PLoS ONE, 6(9). doi:10.1371/journal.pone.0025239.

Twins – identical and fraternal (2001) Better Health Channel. Available at: https://www.betterhealth.vic.gov.au/health/conditionsandtreatments/twins-identical-and-fraternal

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