What Parents Should Know About Fraternal Twins
Fraternal twins develop when two different eggs are fertilized by separate sperm. Learn about characteristics, genetics, and pregnancy with fraternal twins.
Medically reviewed by Yamel Belen, R.N., IBCLCMedically reviewed by Yamel Belen, R.N., IBCLC
Fraternal twins, or dizygotic twins, happen when two different eggs are fertilized by two sperm. "Di" means two and zygotic refers to the zygote, the egg fertilized by the sperm that will develop into an embryo and grow into a baby.
The fact is that twins are quite rare, accounting for only about 3% of live births in the U.S. Of those births, and excluding pregnancies that result from assisted reproductive technology, about 70% are fraternal twins (also known as dizygotic twins); the remainder are identical (monozygotic) twins. Monozygotic twins, or identical twins, come from a single egg and sperm that splits into two after conception.
Fraternal twins share half of their genomes, like any other siblings, and they look completely distinct from one another. Unlike identical twins, they can also be different biological sexes.
Keep reading to learn more about fraternal twins, including common characteristics, risk factors, and what to expect during your pregnancy with two babies.
What Are Fraternal Twins?
Fraternal twins, also called dizygotic twins, develop from the fertilization of two separate eggs with two different sperm. This results in two embryos with their own placentas and amniotic sacs.
Alison Winterroth / Stocksy
How Do Fraternal Twins Develop?
When a person ovulates, a mature egg is released from their ovaries. It travels down the fallopian tube and awaits fertilization with sperm, which can result in conception. Eggs that don't get fertilized shed with the uterine lining during the menstrual period.
Sometimes a person's ovaries release two eggs instead of one. If both eggs are fertilized by separate sperm, you might end up with two embryos—also known as fraternal twins. Each embryo will implant separately in the womb with its own placenta and amniotic sac.
Related: Signs You Might Be Pregnant With Twins
How Common Are Fraternal Twins?
Dizygotic twinning occurs when more than one egg is released during ovulation. This is known as hyperovulation. The incidence of hyperovulation, and in turn fraternal twins, can vary based on several factors, including how the pregnancy was conceived.
According to Sasha Andrews, MD, a maternal-fetal medicine specialist at Pediatrix Medical Group, "fraternal twins are more common in pregnancies conceived with assisted reproduction." This includes ovulation induction medications, intrauterine insemination (IUI), and in vitro fertilization (IVF).
Additionally, "as maternal age and/or the number of children increases, so does the chance of fraternal twins," says Dr. Andrews. The pregnant person's family history is another factor—if their mother or father has a history of twins, then they may be more likely to have twins as well.
Risk Factors for Fraternal Twins
These factors can increase your odds of having fraternal twins:
Using assisted reproductive technology
Family history of fraternal twins
Increased maternal age (specifically over age 35)
Having previously given birth
Taller height
Higher body weight
Smoking before pregnancy
Diet
One study found increased rates of twin pregnancies among those with taller height, higher body weight, and a history of smoking before conception. Another older study suggested that eating dairy products could increase the odds. The University of Rochester Medical Center says Black people assigned female at birth have a higher likelihood of twins, while Asian people and Native Americans have the lowest rates.
Geography can also raise the risk of fraternal twins, with some countries seeing a higher incidence than others. "The commonality of fraternal twins differs across different geographical regions," says registered nurse Shelly Patularu, a twin mom and founder of Twin Mom Guide, a resource for parents of twins.
"Saudi Arabia, Japan, and Vietnam are said to have the lowest rates of twin births in the world. In comparison, Nigeria and Benin in West Africa have the highest twin birth rate." Other factors, such as diet, might affect the twin rates in these locations.
The role of genetics
A tendency toward hyperovulation can be a genetic trait. In this way, fraternal twinning can be hereditary. A parent who has the gene for hyperovulation can pass it down to their child. Then, the child's chances of having twins are increased.
Because parents assigned male at birth carry both X (female) and Y (male) chromosomes, they can also hold the trait for hyperovulation. They can pass it along to their children too, increasing their child's chances of having fraternal twins.
Chances of Fraternal Twins
If there are fraternal twins on both parent's sides, your odds for twins goes up even higher.
The role of fertility treatments
As medical technology has made fertility-enhancing treatments more accessible, the twin birth rate has skyrocketed. While not all treatments for infertility increase your odds of fraternal twins, many do, including.
Femara
Gonadotropins (also known as injectables) like Gonal-F and Follistim
IUI treatment, when used along with fertility drugs
IVF treatment
Interestingly, while IVF might result in fraternal twins (due to the transfer of multiple embryos at the same time), some instances of monozygotic twinning can also occur.
Related: Special Names for Multiple Births
Fraternal Twin Pregnancies: What to Expect
Twin pregnancies tend to be more complex than typical pregnancies, according to Dr. Andrews. "Fraternal twins are more likely to have congenital abnormalities when compared to singletons, and also have a higher risk of fetal growth restriction, or small size at birth," she says. The risk of preterm birth is much higher as well; fraternal twins are often delivered around 36 weeks.
And the birthing parent is at higher risk, too. Maternal disorders such as preeclampsia and gestational diabetes can occur more frequently when carrying multiples. C-section delivery is also more common. But it's important to note that that most people have healthy pregnancies and babies.
Here are some complications associated with fraternal twin pregnancies:
Preterm labor or premature birth
Gestational diabetes
Preeclampsia
Birth defects
Anemia
Low birth weight
Postpartum hemorrhage
Stillbirth
"Prenatal care for twin pregnancies is similar in some aspects, but given the increased risk of complications, more frequent ultrasounds and visits are usually recommended," says Dr. Andrews. "Patients with twins are often referred to a perinatologist, or high-risk pregnancy specialist, for additional counseling and monitoring."
Related: What Are the Different Types of Twins?
Common Characteristics of Fraternal Twins
Unlike identical twins, fraternal twins will always have their own placenta and their own amniotic sac. They can also be different biological sexes, whereas identical twins will always share the same sex.
In fact, you can consider fraternal twins just like any set of siblings, says Patularu. "Because of this, fraternal twins may look similar or very different. They can be very different in size and development, with some fraternal twins reaching milestones at vastly different times."
One thing both fraternal and identical twins may have in common: they can create their own language. "Cryptophasia is the phenomenon of language developed by twins that only the two children can understand," Patularu says. "The secret language usually fades as their vocabulary grows and typically disappears by school age."
How Are Identical Twins Different?
Identical (monozygotic) twins form when one egg is fertilized by one sperm, then it divides into two different embryos during early conception.
Types of Fraternal Twins
Because fraternal twins form from separate eggs and sperm, this can lead to some uncommon circumstances in which the twins have different gestational ages or even different non-birthing parents.
Superfetation
Sometimes, hyperovulation happens a few days apart. After one egg is fertilized and begins to travel to the uterus for implantation, another egg is fertilized by sperm from a later incident of sexual intercourse. The result is fraternal twins who are conceived a few days apart. This is known as superfetation.
Superfecundation
There have even been instances of fraternal twins with different fathers. This occurs when a person releases multiple eggs and has sexual relations with more than one partner. If an egg is fertilized by sperm from one person, and then another egg is fertilized by sperm from a different person, the result is fraternal twins with different fathers. This phenomenon is called superfecundation.
Placenta Fusion
During pregnancy, the placenta provides vital sustenance to the baby. In a multiple pregnancy with fraternal twins, a placenta develops for each baby.
Sometimes, however, the two placentas fuse together and appear to be one single placenta. Having a fused placenta increases the chance of low birth weight and intergrowth discrepancy (in which one twin gets more nutrients and oxygen from the placenta and grows bigger than the other).
However, because they typically have their own placentas, fraternal twins are not at risk for some of the conditions that affect monozygotic twins, such as twin-to-twin transfusion syndrome (TTTS) or monoamniotic twins.
Are My Twins Identical or Fraternal?
Identical twins occur when one sperm fertilizes one egg, and the egg splits in two. They have the same genetics and biological sex, and they might share a placenta and/or amniotic sac. Still, it's not always easy to tell for sure whether twins are fraternal or identical after birth.
Unlike monozygotic or identical twins, fraternal twins may look nothing alike, including having different:
Eye color
Hair color
Stature
Related: What Are Monozygotic Twins?
Sometimes health care providers can tell if twins are identical or fraternal through the placenta. If there's any doubt, parents may choose to do a twin zygosity DNA test in order to determine whether their twins are identical or fraternal.
This is a simple test that can be performed after the babies are born, or later in life. The test consists of a simple, painless cheek swab collected from each twin, allowing a comparison of their DNA profiles. If they're an exact match, they're identical twins.
If you can't make the trip to the doctor to find out, here are some simple pointers that can reveal whether twins are identical or fraternal.
Fraternal Twins (Dizygotic)
Come from their own egg and sperm
Share about 50% of DNA
May look very different
Can be a boy and girl, or the same sex
Identical Twins (Monozygotic)
Come from a single egg and sperm that splits into two
Have same DNA
Look the same
Are always the same sex
Health Risks When Pregnant With Twins
All pregnancies carry risks for the pregnant person. Additionally, twins are affected by your health and habits during pregnancy. Fraternal twins share increased or decreased health risks due to sharing the pregnancy environment.
Being pregnant with twins puts additional demands on your body compared with a singleton pregnancy and increases the risk of the following:
Anemia
Birth defects, including neural tube defects and gastrointestinal and heart abnormalities
Gestational diabetes
Miscarriage
Preeclampsia
Pregnancy-induced hypertension
Premature birth or preterm labor
Coping With a Twin Pregnancy
While it's not possible to completely eliminate the risks that come with having fraternal twins, you can take some healthy steps to reduce those risks, including:
Getting regular prenatal care. Regular prenatal care, ideally with a doctor experienced in twin pregnancies, is essential for catching any problems and receiving proper care.
Eating well. Maintaining a healthy diet and weight can help you feel your best during pregnancy and help ensure your babies are born at a healthier weight.
Staying hydrated. Dehydration is known to trigger premature labor, which is more likely when you're carrying twins, so keep that water bottle filled.
Learning to recognize the signs and symptoms of premature birth. If the signs and symptoms of preterm labor are caught early enough, it may be possible to prevent it so you can carry your pregnancy to term and give your babies a better chance of survival.
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