Table of ContentsView AllTable of ContentsFraternal TwinsAre Twins Hereditary?PrevalenceContributing FactorsIdentical vs. FraternalTypesTesting, Growth, and BirthParentingTwin Studies
Table of ContentsView All
View All
Table of Contents
Fraternal Twins
Are Twins Hereditary?
Prevalence
Contributing Factors
Identical vs. Fraternal
Types
Testing, Growth, and Birth
Parenting
Twin Studies
Whereas identical twins usually look remarkably alike, fraternal twins can look a lot alike, somewhat alike, or even different. They can also be similar or different in personality or inherited health conditions, depending on each twin’s unique genetic makeup and how they respond to their environment.
This article will discuss the biology behind how fraternal twins come about, how they compare to identical twins, gestation, and birth, and associated health risks,
Jill Lehmann Photography / Getty Images
Fraternal Twin Facts
Conception
Research shows that those who have naturally conceived and given birth to fraternal twins have higher levels offollicle stimulating hormone (FSH)in their blood.This hormone helps mature the eggs that are released atovulation.
With higher levels of FSH circulating, there’s a greater chance that more than one egg will be released during ovulation. If sperm meet the egg—or eggs—at the right moment, fertilization and a possible twin pregnancy can occur.
Over the last 40 years, since the explosion of fertility treatments, twin births have just about doubled, although rates are currently declining.Most of the twins born from IVF treatments are fraternal.
What Is Conception?
Genes
Because fraternal twins originate from two separate fertilized eggs, they share only half of their genetic material with each other.Fraternal twins are only as genetically alike as two siblings born at separate times.
Fraternal twins run in families. Having a mother or sister who has given birth to fraternal twins, for example, doubles your chances of conceiving fraternal twins yourself.
Researchers have found that people with variations in two genes,FSHBandSMAD3,have a higher incidence of giving birth to fraternal twins than those without these variations.These gene variations—which can be passed on within families—are responsible for the amount of FSH in the blood and the ovaries' response to it.
About 3% of all live births in the United States are twin births.A large majority are fraternal twins, with only 0.4% of live births being identical twins.
But rates of fraternal twins vary around the world. In Asia, for instance, 6 out of every 1,000 births are fraternal twins. In Africa, the number jumps to 40 out of 1,000.
What Are Mono Mono Twins?
Contributing Hyperovulation Factors
Fertility drugs likeClomid (clomiphene)increase levels of FSH, which, in turn, stimulates follicles to release eggs. But other factors can also play a role in hyperovulation.
Age
Studies from before the era of assisted reproduction show that the chances of having fraternal twins jump fourfold with increasing maternal age from age 15 to 35.
That’s because as you get older, your body naturally produces higher levels of FSH. These higher levels are necessary to help stimulate the release of an egg from a follicle that, due to age, is becoming less and less responsive. If FSH levels are high enough and two good-quality eggs exist, they both may be released.
In one older study looking at over 500 subjects, 105 of them had more than one egg released per cycle. Of these, five were under the age of 30, 45 were aged 30–35, and 55 were over age 35.
Family History
If you’re a birthing parent who has fraternal twins in your family, particularly on your mother’s side, you have an increased chance of having fraternal twins yourself.
If you have a mother or sister who had fraternal twins and you become pregnant, your risk of having fraternal twins is about twice that of the general population.This is likely due to the fact that variations in the genes that lead to hyperovulation are inherited.
Height
One older study comparing the heights of women who gave birth to twins or triplets found that those who carried multiples were, on average, more than 1 inch taller than the general female population.
Identical vs. Fraternal Twins
Identical and fraternal twins have several differences.
The genetic makeup of fraternal twins is equal to that of any two siblings with the same parents (about half). This is because each egg and sperm carries only half of the genetic material from the parent. It is random as to which set contributes to the fertilized egg.
Identical twins receive the same set of X and/or Y chromosomes, and so are typically identified as the same sex at birth. Fraternal twins differ in the X and/or Y chromosome makeup and may be identified as different sexes at birth or the same sex at birth.
While identical twins often look very much alike, fraternal twins can differ in appearance as much as any siblings, including hair, eye, and skin color. Fraternal twins may also develop different hereditary conditions based on the sets of genes they inherited. For example, one may haveDown syndromeand the other not.
Fraternal Twin Types
Superfetation
Superfetation occurs when a second egg is fertilized and implants into the uterus weeks after a previously fertilized egg has already embedded and is developing. This is an extremely rare phenomenon, with only 10 cases being reported in scientifi literature.Superfetation is suspected when there is a size/growth difference between twins.
Heteropaternal Superfecundation
Testing
An ultrasound showing two developing babies with distinct placentas and amniotic sacs can indicate that you’re having fraternal twins. Twins that are different sexes at birth are fraternal. Twins of the same can be either fraternal or identical.
Growth
A uterus is only so big, and with two developing fetuses vying for oxygen, space, and nutrients, it’s not uncommon to see growth problems with one or both twins. These growth problems can be due to what’s known as intrauterine growth restriction (IUGR).
IUGR is a condition in which a fetus is not as big as would be expected for its gestational age (i.e., how far along the pregnancy is). It occurs in 25%–35% of twin pregnancies.
Babies born small can have a host of health issues, including problems with:
Birth
Twin pregnancies have a high likelihood of ending in a preterm delivery, which is delivering before 37 weeks of pregnancy (a full-term pregnancy is 40 weeks). Research shows that twins account for 20% of all preterm deliveries, 60% of which occur before 37 weeks and 10% before 32 weeks.
Because of being born early, before organs and body systems are fully developed, twins have a 5 times higher risk of early infant death, as well as many other health issues—from problems eating to breathing.
What You Need to Know About a Twin Pregnancy
Parenting Fraternal Twins
Parenting your twins won’t necessarily be like parenting their siblings. Experts advise that you:
Future of Medicine With Fraternal Twin Studies
Both fraternal and identical twin studies give scientists an inside look at how shared genes and environment can impact the development of things like personality, physical traits, and disorders.
These studies can help determine how much of one’s characteristics or disease risks are inherited and how much are nurtured through a shared upbringing.
For example, scientists know that when one identical twin hasschizophrenia(a serious mental disorder that can impact one’s sense of reality), the other has a 50% chance of also developing the condition.But when a fraternal twin has the condition, the other twin has just a 10%–15% of also having it.
This tells scientists that the condition is largely (although not completely) genetic in nature.
Many academic centers maintain twin registries. Some current studies are looking at twins with early-onset diabetes, the skin condition psoriasis, and rheumatoid arthritis, among other conditions.
Summary
There are a variety of factors that increase the likelihood of conceiving fraternal twins. These include fertility drugs, genetic predisposition, and increasing maternal age.
Fraternal twins share 50% of their DNA, making them about as genetically similar as any other siblings. To determine whether twins are fraternal or identical, ultrasound findings during pregnancy or DNA tests after birth can be done.
Twin pregnancies have increased risk for intrauterine growth restriction, preterm delivery, and cesarean delivery.
A Word From Verywell Health
Most twins born in the United States are fraternal. But even though they are born together, the twins are really no more similar than regular siblings. They may look different, act differently, have different talents and interests, or—like siblings in general—they may share traits.
Because they must share a womb, competing for space and nutrients, fraternal twins can be born with some health complications. But with good and regular prenatal care, you can help minimize some of those risks.
FAQs
Frequently Asked QuestionsBecause they share some, but not all, of their DNA, dizygotic twins may have the same or different blood types.Monozygotic twins, or identical twins, on the other hand, typically have the same blood type.Basically, it’s that they only share about 50% of their DNA. One twin may have inherited blue eyes from one parent, for instance, while the other got brown eyes from the other parent.Genetically speaking, fraternal twins are the same as siblings born at other times.For the most part, fraternal twins develop with two separate placentas, while identical twins share one. Your doctor should be able to see this on an ultrasound. If they are identified as two different sexes at birth, it is highly likely they are fraternal twins (twins of the same sex can be either fraternal or identical).But the only way to definitively tell if twins are identical or fraternal is DNA testing after birth.
Frequently Asked Questions
Because they share some, but not all, of their DNA, dizygotic twins may have the same or different blood types.Monozygotic twins, or identical twins, on the other hand, typically have the same blood type.
Basically, it’s that they only share about 50% of their DNA. One twin may have inherited blue eyes from one parent, for instance, while the other got brown eyes from the other parent.Genetically speaking, fraternal twins are the same as siblings born at other times.
Basically, it’s that they only share about 50% of their DNA. One twin may have inherited blue eyes from one parent, for instance, while the other got brown eyes from the other parent.
Genetically speaking, fraternal twins are the same as siblings born at other times.
For the most part, fraternal twins develop with two separate placentas, while identical twins share one. Your doctor should be able to see this on an ultrasound. If they are identified as two different sexes at birth, it is highly likely they are fraternal twins (twins of the same sex can be either fraternal or identical).But the only way to definitively tell if twins are identical or fraternal is DNA testing after birth.
For the most part, fraternal twins develop with two separate placentas, while identical twins share one. Your doctor should be able to see this on an ultrasound. If they are identified as two different sexes at birth, it is highly likely they are fraternal twins (twins of the same sex can be either fraternal or identical).
But the only way to definitively tell if twins are identical or fraternal is DNA testing after birth.
23 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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