Congratulations, it’s … two!

Is there a moment more surreal than finding out you’re carrying twins? After the shock wears off, many questions will follow.

Identical or fraternal? Boys or girls? How did this happen? One thing that’s for sure… family life is going to be double the fun!

Identical twins are the least common types of twins — making up about one third of all twin pregnancies. They also carry the most mystery, misunderstandings, and unanswered questions.

We will dive into what makes twins identical and some of the most fascinating things to know about them.

Identical Twins Definition

Twins are two babies born from the same pregnancy.

Twins that are identical occur when one sperm fertilizes one egg. The resulting embryo splits early in development, and creates two separate embryos.

These types of twins are also known as monozygotic twins.

How common are twins?

The likelihood of conceiving twins that are identical is about 3 to 4 in 1000. So it’s a relatively rare thing to happen and there’s still a lot to learn about these special sets of human beings.

#1: Are twins genetic?

Fraternal (dizygotic) twins do run in families. Interestingly, the genetics of fraternal twins comes from the mother’s side.

Fraternal twins happen when a woman releases two eggs at ovulation (hyperovulation). Ovulation is controlled by our genes. Some women are more likely to hyperovulate, because of their genes and this is passed down on the female side. Fertility drugs and assisted conception can also increase the chances of fraternal twins.

Having twins that are identical is not genetic, as they are the result of a single fertilized egg splitting after fertilization. Since embryo splitting is a random event and happens by chance, there are no genetics involved. So twins that are identical don’t run in families.

#2: Identical vs fraternal twins

Identical twins start out as a singleton pregnancy, and are also known as monozygotic twins. If we break this word down, ‘mono’ means one, and ‘zygote’ means fertilised egg. One fertilised egg divides and split into two separate babies.

This is the big difference between fraternal (dizygotic) and identical (monozygotic) twins. Fraternal twins result from two separate eggs fertilised by two different sperm.

#3: How identical twins form

It’s not completely understood why or how a single fertilized egg happens to divide. The timing of this event, however, is crucial.

The number of days between fertilisation and the splitting of the fertilized egg determines some important differences in a twin pregnancy:

Day 0-3: Embryos develop 2 amniotic sacs and 2 separate placentas
Day 4-9: Embryos will share a placenta but form in separate sacs
Day 9+: When the split occurs after day 9, the twins share a placenta and a sac.

The earlier the split, the more likely the twins will grow inside their own separate sac with their own separate placenta. This reduces the risk of complications that arise from sharing these structures.

Splits that occur later than the first 7 days post-fertilisation are more likely to result in rare instances of mirror image twins or conjoined twins.

#4: What makes identical twins identical?

Prepare to have your mind blown: identical twins are not always identical!

How is it possible that people formed from the same cells aren’t exactly the same?

We’re not just talking about being able to tell twins apart from their outward appearance. While these twins share their looks, twins that are identical can also have different personalities and different susceptibility to disease. This takes us back to the age-old question of ‘nature versus nurture’ and the roles both play in development.

As researchers explain, exposure to different environments can cause changes and adaptations in human gene expression.

But geneticist Carl Bruder and his team went deeper, and found some differences occurred even at the genetic level in monozygotic twins.

Bruder, in Scientific American (2008) says, “I believe that the genome that you’re born with is not the genome that you die with—at least not for all the cells in your body”.

#5: Do identical twins have the same DNA?

Even though we’ve just established identical twins can have differences, by definition these twins start with the same DNA.

Monozygotic twins are formed from the same egg cell and same sperm cell. Their genetic coding comes from exactly the same source. This is most evident in outward characteristics such as hair colour, eye colour, and body shape.

Until fairly recently, scientists maintained that monozygotic twins had 100% the same DNA throughout their lives.

It’s since been shown that when twins start to develop as separate individuals, gene differences can begin to emerge while they are still in the uterus.

#6: Do identical twins have the same fingerprints?

A case in point for differences in identical twins: their fingerprints don’t match!

The whorls, arches, and loops that make up fingerprint ridges are fully formed by 6 months of fetal development. Fingerprints are unique to each individual human being, even monozygotic twins.

We all know how fingerprints are used to prove identity for many reasons, including in criminal investigations. Check out this unusal story of an identical twin who thought he could get away with murder until his fingerprint solved the case.

#7: Can identical twins be boy and girl?

Usually (99.99% of the time) identical twins are also the same sex. Two boys or two girls. You’d expect it to be that way, as they start out with the same genetic code.

It’s fascinating to find out boy and girl monozygotic twins are possible, although extremely rare. This only happens when one of the embryos starts as a male zygote, and then develops a chromosomal condition called Turner Syndrome.

#8: Identical twins ultrasound

Most people can determine twins are identical based on how alike they look. Before they’re born, though, it’s not always easy to tell whether you’re having identical or non identical twins.

Usually twins are detected by ultrasound at around 11-14 weeks. An identical twin ultrasound will look for:

Ultrasound diagnosis of 2 babies in one amniotic sac (monoamniotic)
Diagnosis of TTTS (Twin to Twin Transfusion Syndrome), where babies share a circulatory connection
Blood testing. Twins that are identical always share the same blood type, and fraternal twins sometimes do
Placenta analysis. One shared placenta most often indicates the twins are identical, but two placentas can occur with identical or fraternal twins
In the case of in vitro fertilisation, if there are more embryos present than were implanted, this could be a case of spontaneous monozygotic twins
Physical similarities. Babies that look remarkably alike is often an indicator of identical twins, but isn’t conclusive.

Conclusive testing for identical twins (monozygosity):

Twin zygosity test (a DNA test after the babies are born, where a cheek swab is sent to the lab).

#9: Semi-identical twins

You’ve probably heard of identical and fraternal twins, but another possibility is semi-identical twins. They are extremely rare: experts say only two cases have ever been identified.

Semi-identical twins occur when one egg is fertilised by two sperm, at exactly the same time, before it divides. Because it’s uncommon for both embryos to survive, semi identical twins are very rare. They can share 50-100% of their genes.

If you have found out that a set of twins is about to come into your life, congratulations! Twins will certainly bring double the fun, as well as double the number of odd questions from strangers.

With these fascinating facts under your belt, you should at least be able to answer some of those questions with confidence.

Be sure to check out Pregnant With Twins? 8 Interesting Facts About Twins for more amazing facts about twins.