What You Need To Know About Twins
One of the first things pregnant people often wonder about is whether or not they will have twins. But according to the Centers for Disease Control and Prevention (CDC), the likelihood of a pregnant person having twins is only about 3%. In fact, for every 1,000 births in the United States, only 31.2 of them are twins.
And though twinning is a rare occurrence, many people are still fascinated by the possibility, says Laura Purdy, MD, MBA, a board-certified family physician. “Understanding the different types of twins can give parents insight into the fascinating variations that may emerge during gestation.”
So, whether you are pregnant with twins or just curious about the many different types of twins, keep reading to learn more. Below you will find information on nine different types of twins, from fraternal to identical and beyond.
1. Identical Twins
Identical twins, also known as monozygotic twins, occur when one fertilized egg separates into two embryos and later becomes two babies, says Dr. Purdy. Because the babies share the same egg and sperm, their genetic material is identical. In other words, they are assigned the same sex at birth and typically have the same physical features like skin tone and hair color.
“Though identical twins occur less frequently than fraternal twins, they still account for roughly one-third of twin pregnancies worldwide,” Dr. Purdy says.2 “What’s more, the presence of identical twins is unrelated to factors like maternal age or fertility treatments.”
Interestingly, there are times when identical twins will sometimes look slightly different, even though they have the same genetic makeup. This phenomenon is usually related to environmental factors like their positioning in the womb. Other factors that can help differentiate identical twins from one another include illnesses and lifestyle choices. These experiences can result in different genes being turned on and may cause the twins to look and act differently.
- Fraternal Twins
Fraternal twins, commonly referred to as dizygotic twins, form when two separate eggs are fertilized by different sperm, Dr. Purdy says. “Factors including maternal age, family history of fraternal twins, and fertility treatments may increase your odds of fraternal twin birth.”
This type of twinning is about twice as common as identical twins and represents about 70% of all twin gestations. Fraternal twins are believed to result from hyperovulation, which occurs when more than one egg is released during the menstrual cycle.
Other factors that can increase the likelihood of fraternal twins include ethnic background, diet, body type, and the number of other children you already have. There also is some evidence that assisted reproductive technologies such as in vitro fertilization (IVF) is associated with an increased frequency of fraternal twins.
- Conjoined Twins
Conjoined twins happen when there is a twinning, but the division isn’t completed, and as a result, the twins are physically conjoined or connected, explains Kristin Mallon, C.N.M., M.S., RNC-OB, a board certified nurse midwife, certified obstetrical nurse, and co-founder and CEO of FemGevity Health.
“This type of twinning is extremely rare,” she says. “It happens in about 1.5 to 100,000 births worldwide. It also happens to affect female twins more often than male twins.”
With conjoined twins, they have a body part that is fused together and they are labeled based on the location of their bodies, Mallon adds. Prognosis for conjoined twins depends on individual circumstances and extent of their physical connection. In some cases, surgical separation may be used.
People who are pregnant with conjoined twins are at a higher risk of serious complications and their babies are more likely to be born prematurely. Due to the fact that the babies are conjoined twins, they are delivered via C-section. What’s more, the rate of stillbirth is around 60%.
- Parasitic Twins
A parasitic twin is a rare condition that is closely related to conjoined twins. In this situation, a baby is born with an underdeveloped twin attached to its body. Because they are connected, these babies often share organs.
Typically, a parasitic twin develops when one of two twins fails to develop fully and becomes dependent on their counterpart, Dr. Purdy says. “Parasitic twins don’t form because of a maternal age factor, fertility treatments, or genetic traits. Rather, this phenomenon results from random developmental anomalies.”
This type of twinning is extremely rare with only about 0.1 babies born per 100,000 births. Prognosis for parasitic twins depends upon their degree of physical dependence and any associated complications, Dr. Purdy says. Surgery may be considered if it will safeguard the twins’ wellbeing.
- Superfetation
Superfetation occurs when a person is already pregnant and still ovulates, resulting in a twin, says Mallon. “This second ovulation can happen very quickly once the initial pregnancy occurs and can be very hard to differentiate between normal twin pregnancies where ovulation happened at the same time.”
This type of twinning is rare because of the way reproduction works, she adds. “Reproduction is programmed in such a way that once a person has ovulated, the [chances of] ovulation happening a second time is highly unlikely. And, certainly, once a person is pregnant, ovulation happening during pregnancy is extremely rare.”
Research suggests that superfetation happens once out of a few million pregnancies. Additionally, there have been fewer than 10 cases in the world. Some experts speculate that it may occur when a person has a double uterus.
“Superfetation [also] does not depend on maternal age, fertility treatments, genetic factors, or environmental influences,” Dr. Purdy says. “Rather, it results from an unexpected hormonal and physiological condition.”
- Mirror Image Twins
Mirror image twins are a type of identical twins, says Mallon. “This is a type of twinning in which the embryonic division occurs, and the twins have mirrored features.”
For instance, if one twin is right handed, the other might be left handed, she adds. “Or if one twin has a dimple on the right cheek the other will have a dimple on the left cheek.”
This phenomenon is thought to be caused by the egg splitting later than it would in a more traditional identical twin pregnancy. This extra time before splitting means that the fetuses had long enough to develop a right and a left side. Researchers estimate that 25% of identical twins are actually mirror image twins.
This type of twinning is due to random variance in physical development processes, Dr. Purdy says. “Parents should also know that mirror image twins, while having differences such as birthmarks on either side or hair whorls in different directions, still fall under identical twin status based on genetic relationship.”
- Heteropaternal Superfecundation
When a pregnant person gets pregnant while already carrying another pregnancy, this is known as superfecundation, Dr. Purday says. If the babies have different fathers, then this is called heteropaternal superfecundation. “This type of pregnancy is extremely rare and only a few cases have been recorded worldwide.”
Most recently, a case was reported in Columbia after a paternity test showed that the twin boys had different fathers. Researchers speculate that the person became pregnant by one father and then had sex with another person. Because an egg was released and then fertilized by that second person, two pregnancies occurred by two different fathers.
- Polar Body Twins
Polar body twinning, or half-identical twins, is thought to occur when an egg splits and each half is then fertilized by a different sperm, Dr. Purdy says. This results in twins who look very much alike but do not share 100% of their DNA. “Polar body twins would likely result from an isolated cell event rather than maternal age, fertility treatments, or genetic influences.”
Right now, the concept of polar body twins is primarily a theory, she adds. “There are no documented reports having occurred thus far.”
- Semi-Identical Twins
In 2006, a new type of twinning was discovered—semi-identical twinning. In a report published in the Journal of Human Genetics, researchers indicated that they had discovered a set of twins that had characteristics of both fraternal twins and identical twins. In other words, they were identical on the mother’s side, but only shared half of their father’s genes.
Then, in 2019 a second case was published in the New England Journal of Medicine. Researchers discovered that in a 28-year-old’s pregnancy her twins shared a placenta, appearing to be identical twins. But the 14-week ultrasound revealed that they were different genders—making it impossible for them to be identical twins.
Though extremely rare, this type of twinning may develop when two sperm fertilize a single egg, which then splits. This process is different from identical twins which form from a single fertilized egg that splits into two and fraternal twins, which are formed when two separate eggs are fertilized by two different sperm.
The Bottom Line
More than 70% of twin pregnancies are fraternal twins, with the majority of other types of twins being identical. Twins—like conjoined, parasitic, and polar body twins—are extremely rare. Most of the time, researchers do not know why these types of twinning take place.
If you are pregnant with twins, it is important to recognize that twin pregnancies have a higher risk of complications. For this reason, you need to make sure you are keeping your prenatal care appointments and communicating regularly with your pregnancy care team and health care provider. They can advise you on how to handle any concerns.
