“Are they twins?!” The lady at the grocery store stops you on the way in, admiring your boy-girl twins.

Yes they are!” You know what question is coming next.

“Are they identical?” You start to consider how to best explain it to her, but then you realize you’re on borrowed time with two toddlers in a grocery store. “Nope!” You quickly push the shopping cart into the store.

If you are a parent of multiples, or a twin/triplet yourself, that second question appears on a regular basis. Whether you have boy-girl twins, or obviously fraternal twins, most people just don’t understand how twinning happens, and so the questions they ask can be nonsensical and frustrating if you hear them over and over again. Instead of getting annoyed, remember how little you knew about twinning before you found out you were expecting multiples, then send them a link to this post!

Zygosity: Identical or Fraternal?

Most laypeople classify twins into one of two categories: “twins that look alike” and “twins that don’t look alike.” Twins that look alike are typically assigned as identical, and twins that don’t look alike are assigned as fraternal. Some people may understand that the twins’ physical appearance correlates with similarities or differences in their genetic make-up. While this is an oversimplification of the relationship between genotype (the genetic code) and phenotype (the physical characteristics that result from the genetic code), it’s the basis of zygosity as it relates to classifying twins as identical or fraternal.

Development of Dizygotic versus Monozygotic twins.

Egg cells and sperm cells contain one gene for each trait (eye color, height, predisposition to certain diseases, etc.). Therefore, when an egg and sperm fuse during fertilization, the resulting zygote ends up with two copies of every gene. The two copies of a particular gene may be the same or slightly different—this is determined randomly. It is this random genetic variation in the gene pairs that makes individuals different from one another, even if they have the same parents. 

Back to zygosity. A zygote is a single cell that is made by fusing the egg and sperm during fertilization. If one sperm fertilizes one egg, the result is one zygote. If the mother releases more than one egg at a time during ovulation, there is a possibility that two sperm will fertilize two eggs, resulting in two zygotes. Obviously, two zygotes results in two embryos that develop into two babies. These two babies are dizygotic (di = two, zygotic =zygote), or fraternal, twins. Because each zygote contains their own random variation of gene pairs, fraternal twins may look quite different from one another and are, in fact, no more genetically similar than singleton siblings. This mechanism of fraternal twinning is the hereditary type of twinning. The actual trait of hyper-ovulation (releasing more than one egg at a time during ovulation) is genetically determined. A woman who is a fraternal twin may very likely hyper-ovulate and have her own set of fraternal twins. I learned this first-hand as a fraternal twin who ended up pregnant with twins (the second most common question I get is: “But doesn’t it skip a generation?” Not always! Keep reading). Now my boys look pretty identical and I have not had their zygosity tested, but it is worth knowing that fraternal twins can sometimes look a lot alike, and maybe even exactly alike. Have you ever seen a picture of someone’s older sibling who looked just like them? Or maybe you look exactly like your dad when he was your age—genetics and inheritance are wonderfully mysterious things.

So what is an identical twin? Identical twins are called monozygotic (mono = one, zygotic = zygote), meaning they are two separate fetuses that developed from one zygote. How does that happen? We know that after fertilization the single-celled zygote splits into two zygotes with identical copies of each gene pair. So unlike dizygotic twins, monozygotic twins are genetic clones of each other. This is why they often look identical (though some can look quite different). There are many things we don’t understand about how monozygotic twins form: what triggers the zygote to split? How can some identical twins look different or have different personalities? There is some anecdotal evidence that identical twinning runs through the male line, and therefore might have something to do with sperm physiology, but no one really knows. Identical twins are much rarer than fraternal twins, occurring in approximately 3-4 out of every 1,000 births and dizygotic twins occurring at about twice that frequency1.

Zygosity Testing

Zygosity testing is a simple DNA test that determines how alike the twins’ genomes are by assessing a select number of genetic sequences for similarities. For most commercial zygosity tests, cheek cells are collected from each twin using a cotton-tipped applicator stick (or Q-tip) and mailed to a laboratory that conducts molecular testing. Each of the cheek cells collected contain a nucleus with DNA. The DNA is extracted in the lab and specific gene sequences are amplified by a method called Polymerase Chain Reaction (PCR). Up to fifteen specific, short gene sequences are analyzed (this is also called gene fingerprinting). Identical twins will show no differences between these sequences, while fraternal twins show two or more differences3. This is the same methodology used in forensic science to analyze DNA of suspects or victims of a crime.

Chorionicity and Amniocity: Di/Di, Mo/Di, or Mo/Mo?

Development of the chorion and amniotic sacs during different types of twin pregnancies.

When I tell people that I don’t know for sure if my twins are identical or fraternal, I often get the question, “Can’t the doctors tell from the placenta?” Usually the people asking this question know a little more about twinning than the general population. But the answer is still, “Not always.” How can the placenta tell you anything about your twins’ zygosity? Let’s start with some definitions. The membranes that surround the fetus are comprised of two layers: the chorion and the amnion. The chorion is the outermost membrane and contacts the uterus during implantation—this point of contact will become the placenta. The amnion is the innermost layer and surrounds the developing fetus without making contact with the outside environment. The amniotic sac contains fluid that cushions and protects the fetus while providing metabolic support. Therefore, chorionicity refers to the number of placentae present in a twin pregnancy and amniocity refers to the number of amniotic sacs. In a singleton pregnancy derived from one zygote, the fetus will have one chorion and one amnion (one placenta and one sac). In the case of dizygotic twins derived from two zygotes, each fetus will develop its own placenta from the chorion and its own amniotic sac; they are therefore referred to as dichorionic/diamniotic twins (two placentas/two amniotic sacs; one for each twin), or Di/Di twins.

The formation of the placentae and amniotic sacs in identical twins is more variable and the exact structure depends on when the zygote splits into two separate zygotes. If the split happens soon after fertilization (within 2-3 days2), the chorion and amnion will develop separately and the fetuses will be Di/Di and, therefore, indistinguishable from dizygotic twins. A zygote that splits between 4 and 8 days after fertilization can no longer divide the chorion, but the amnion can still be divided2. These twins are referred to as Monochorionic/Diamniotic twins (one placenta/two amniotic sacs), or Mo/Di twins. Zygote splitting that occurs between 9 and 12 days after fertilization is rare, and both the chorion and the amniotic sac are unable to divide at this time, resulting in the twins sharing both2. These types of twins are monochorionic/monoamniotic (one placenta/one amniotic sac), or Mo/Mo twins. Of course, zygote splitting can occur even later between 12-15 days, which will result in incomplete division of the zygotes or, conjoined twins2.

Four Common Twin Myths and How to Explain Them

Now that we know a bit more about how twinning happens and how the placentae and amniotic sacs form, let’s look at some common twin myths and how to explain them away.

1. Boy/Girl Twins can be identical: This is just a misunderstanding about how zygosity, genetics, and biological sex determination are interrelated. Biological sex is determined genetically by the presence of two X chromosomes (for female anatomy) or a XY chromosome combination (for male anatomy). Remember, identical twins have identical genomes, so if you have one twin with a XY chromosome pair, the other twin has to have one too. And vice versa, if one twin has a XX pair, the other one can’t have a Y chromosome. All identical twins are same-sex twins, and all boy/girl twins are fraternal. Fraternal twins are the only type of twins that can be either same-sex or opposite-sex. Of course, there are some exceptions, but those are rare and beyond the scope of this post.

2. You can tell you’re having fraternal twins if there are two amniotic sacs and two placentae: In monozygotic twins, if a zygote splits early after fertilization (within 2 to 3 days), the resulting identical twins will develop in separate sacs and each have their own placenta, making them indistinguishable from fraternal twins (if they are the same sex). In this case, zygosity testing that assesses each twin’s genetic fingerprint is the only way to know for sure. Obviously Di/Di, Boy/Girl twins are always fraternal and do not require zygosity testing.

3. Identical twins always look the same and fraternal twins always look different: Physical appearance can be hard to predict even if you know something about the person’s genome. Fraternal twins’ genomes will always be different from one another because the process of fraternal twinning includes shuffling of genetic material—the same as in separate singleton pregnancies. Identical twins’ genomes will always be the same because the process of identical twinning produces genetic clones. All of that being said, epigenetic factors (i.e., environmental factors that influence the expression of genes) can result in identical twins with differences in their physical appearance, and the random inheritance of similar physical traits can result in fraternal twins looking quite alike.

4. Fraternal twinning always skips a generation: It is true that a woman who gives birth to a male fraternal twins should not count on having a set of twin grandkids. Male fraternal twins are no more likely to have fraternal twins than the rest of the population (they do not ovulate). That being said, a male twin’s singleton daughter may inherit the hyper-ovulation trait and will therefore be more likely to have a set of fraternal twins of her own, making it seem as though a generation was “skipped”. On the other hand, a mother who has a girl fraternal twin may very well pass along the hyper-ovulation trait to her, making her more likely to get pregnant with fraternal twins and resulting in two successive generations of twins.

Hopefully this simplified guide to twinning gives you some talking points (or at least some patience) the next time someone outside of Twin World starts asking you questions that don’t quite make sense. Twinning is a complicated process with lots of variables and lots of factors that we don’t yet understand….and I didn’t even mention tripleting.

About the author

Brooke Dubansky, PhD is TLC’s Dallas associate and the mother of 2 year old twin boys, who may or may not be identical. She is a fraternal twin herself, and so she assumed her twins would also be fraternal, but now has her doubts since no one can seem to tell them apart. She earned a PhD in Anatomy from Louisiana State University and is a certified Histotechnologist. She has an educational interest in breast anatomy and lactation, among other healthcare related topics. You can reach Brooke at brooke@twinloveconcierge.com for more details on our Dallas services.

1 Help Me Understand Genetics: Genetics and Human Traits. 2019. Genetics Home Reference (https://ghr.nlm.nih.gov).

2Fuchs KM & D’Alton ME. 2018. Obstetric Imaging: Fetal Diagnosis and Care, 2nd Edition. Elsevier, Inc., Pp. 642-645.

3Identiy Testing: Zygosity Testing. Molecular Genetics Laboratory, Hospital for Sick Children. www.sickkids.ca