Sometimes good things are in pairs. Two peas are better than one. A job well done deserves two thumbs up. The ultimate two-for one deal: twins.
Fraternal and Identical twins are the most common types of twins. What’s the difference between fraternal and identical twins? Are there other types of twins as well?
We spoke with Ob/Gyn Julian Peskin MD, who is also a twin, about the common types of twinning, rare types, and anything else you may have been curious about.
What is the difference between identical and fraternal Twins?
To understand how twins are born, let’s first look at the basic structure of embryos.
Ovulation releases one or more egg. A zygote is formed when an egg is fertilized with sperm. The zygote grows quickly into a cluster of cells which travels through the fallopian tubes and implants itself in the uterus. The zygote continues to grow, and forms a Placenta that provides nutrients to the embryo.
- Fraternal Twins are created when two eggs (typically from the same donor), fertilized by different sperm, in the same ovulation period. The twins are derived from two different zygotes and therefore have separate genes.
- Twins who are identical result from a zygote splitting in two. The twins are identical because they come from the exact same egg and sperm.
Fraternal twins
You might think that one egg is released for each ovulation. It’s not true. Some people release more than two eggs at once. This is called superovulation.
Fraternal twins are created when two sperm fertilizes two eggs. Dizygotic twins, or fraternal twins, are twins that come from two zygotes.
How similar are fraternal and identical twins?
The genetic similarity of fraternal twins is no greater than that of any other pair who shares biological parents.
Dr. Peskin explains that each egg and sperm contains different genes. Each egg and sperm contains a combination of genes that parents have passed down to them.
One twin may have blue eyes, while the other has brown eyes and no dimples.
The sex of fraternal pairs can be different. They could be two boys or two girls or even one boy and one female.
Placenta development
The uterus will produce two placentas that will nourish the twins until they are born. These placentas may fuse.
Some people say that if only one placenta is present, then they are identical twins. But this is not always the case. “It can become cramped in the uterus and cause those two placentas to start growing together,” Dr. Peskin says. It’s actually two separate placentas, with separate blood vessels. But it may look like just one.
How common are they
Fraternal twins occur more often than identical twins. What’s fascinating, says Dr. Peskin, is the fact that fraternal pairs are more common in certain parts of the globe than others.
Researchers have found that the highest rates of twin births worldwide occur in Africa and North America. In these regions, 17 of every 1,000 babies are twins.
Fun fact: the Igbo Ora community of Southwest Nigeria has one of the highest rates in the world for fraternal twins. Researchers have found that fraternal pairs account for 45 of every 1,000 births within the community.
In South America and Asia, twins are more rare — about nine out of 1,000 births.
The rates of identical twins are the same everywhere in the world. “The rate of fraternal pairs varies greatly,” says the researcher. The reason is still unknown, but it is believed that you are genetically predisposed towards superovulation. This is necessary for fraternal doubles to form.
Who is more likely to have fraternal identical twins?
You may have heard that fertility treatments such as in-vitro fertilization and the fertility drug clomiphene increase your chances of having fraternal identical twins. It’s true. But there are other factors. Dr. Peskin states that people are more likely than others to superovulate when:
- They are older than 35.
- They have a BMI greater than 30.
- They have already given birth.
- The twins were fraternal in the past.
- They are fraternal twins.
Identical (monozygotic) twins
Monozygotic twins is another name for identical twins. They are identical twins, and they’re also called monozygotic twins. Scientists don’t yet know the cause of this split. The result is that two embryos have the exact same genetic material. It’s because the sperm and egg are the same.
The same gender will be assigned to identical twins at birth, except in the case of an identical twin with ambiguous genitals.
The two may not be identical
The appearance of identical twins is very similar. Most siblings are more similar. Dr. Peskin says that “identical” twins can still have physical differences.
Dr. Peskin explains that there are other factors besides genes which can affect your appearance. The lives identical twins live can have a significant impact on their appearance. It doesn’t matter if one twin smokes or spends more sun time, exercises more or is more stressed.
You may see differences between identical twins even at birth. In the womb, small genetic mutations may occur. One twin might have a mole, or a birthmark to help you identify them. Maybe one twin got more blood than the other from the placenta, and so is born bigger.
How common is it to have identical twins?
Fraternal twins are more common. About 3-5 out of 1,000 births are identical twins. Fraternal twins are more likely to be passed down through families than identical twins.
Pregnancy complications
There’s a common myth that fraternal and identical twins each have a placenta. Dr. Peskin disagrees. Only after birth can you tell if twins are identical, fraternal or not.
It’s possible that identical twins can have either one or two placentas. The split of the zygote will determine how many placentas there are. If the split occurs earlier in development, about three to four days after fertilization they will create two separate placesntas. If the split occurs later, then they could share a placenta.
The issue with a monozygotic placenta is that if blood vessels are shared, it’s known as twin to twin transfusion syndrome and can lead to one twin receiving much more blood than another. One twin can become very large and the other smaller or underdeveloped.
It is for this reason and other reasons that twin pregnancy is considered a higher risk than singletons. It is recommended that women who are pregnant with twins or other multiples have additional prenatal tests and appointments in order to ensure healthy development of the fetus.
Rare twins
Other types of twins can also occur, but they are rare. Because they are so rare, these twins get a lot of attention in the media and on news. Think of scenarios you would see more often on a medical drama than in reality.
Conjoined Twins
Conjoined Twins are very rare. They occur when one zygote starts to split but does not completely separate. Two identical twins share at least one organ.
Dr. Peskin says that if the egg splits around day seven after fertilization, it is possible for twins to share organs.
Conjoined Twins can be joined in many ways, including at the chest, belly, or head. Dr. Peskin says that some conjoined pairs are separated today, but not all. Parents can make the decision to separate conjoined identical twins in consultation with other people and their medical team.
According to estimates, conjoined twins occur once every 50,000 births.
Twin parasites
Conjoined Twins are two fetuses who are fully developed and connected.
Parasitic Twins are another type of twin in which one twin is fully-developed and the other twin is underdeveloped. Underdeveloped fetuses are nonfunctional. It is often not able to survive pregnancy. It may appear that one person has extra limbs, or organs.
Conjoined twins can develop in a similar way to parasitic twins where the zygote does not fully separate. They may also be formed from two fertilized eggs fused together. The twins will remain attached to one zygote, which stops growing and grows no more on its own.
It is estimated that parasitic twins affect less than 1 out of 1 million births in the world.
Twins with different gestational age
A pregnant woman will not ovulate during her pregnancy. The hormonal changes that occur during pregnancy almost always prevent another egg from forming and stop another embryo from forming.
In rare circumstances, an egg may be released during pregnancy. This egg can be fertilized and grown.
This is called superfetation. Two fetuses are born at different developmental stages because they were conceived independently. Only 10 cases of superfetation have been confirmed. Although they are of different gestational ages, the twins tend to be born at the same time.
Twins of different fathers
It is possible that twins can have two different fathers in cases of superfecation. The eggs may be released at different times and could therefore be fertilized with sperm of two separate partners.
Half-identical twins
It’s common knowledge that one egg and one single sperm is required to create a child. In rare cases, two sperm can fertilize one egg. There are only two known cases.
In a case study published in 2019 , a pair of “half-identical” twins were described. Because they had a shared placenta at birth, doctors initially assumed that they were identical twins. When an ultrasound revealed that they had different genitalia more testing was performed.
Scientists have found that the twins are genetically identical. They share the same DNA with their mother, and they also share 78% DNA with their father.
According to this theory, two different sperm fertilized an egg and then the zygote divided. The twins are genetically identical but came from the same egg.
Semi-identical twins
You might have heard of another type twinning called semi-identical or polar twins. Researchers claim that there have been no cases identified of this type of twins, and we do not have a test to detect them. The idea that these twins could exist is a theory rather than something we can confirm.
Unfertilized eggs divide. This is a normal process of ovulation. This results in an egg and a small offshoot, called a “polar body”. The polar body usually is underdeveloped and disappears.
The polar body could, in theory be fertilized with the egg if it is strong enough. The result will be twins who are somewhere between identical and fraternal. This is because they will come from two different sperm but the same egg.
Many mammals have twins, triplets or even multiples of higher levels. European hedgehogs have up to nine babies at once. Humans are more a species that tends to have one child at a given time. Even though twins can be formed in many different ways, it’s most likely that you will meet fraternal or identical twins.
