The Unique Biology of Fraternal Twins

Fraternal twins, also known dizygotic (DZ) or nonidentical twins, are twins that result from two separate eggs released from the ovary and fertilized by two different sperm cells within the same menstrual cycle. The two fertilized eggs implant in the uterus and develop during one pregnancy.

Identical, or monozygotic, twins, on the other hand, develop from one egg that’s fertilized by one sperm. The fertilized egg (zygote) then splits after fertilization.

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,

Fraternal Twin Facts

Conception

Fraternal twins are thought to be a byproduct of what’s known as hyperovulation. Normally, the ovaries release one egg per menstrual cycle. But in some cases, two or more eggs may be released, increasing the chances of having a multiple birth. 

Research shows that those who have naturally conceived and given birth to fraternal twins have higher levels of follicle stimulating hormone (FSH) in their blood. This hormone helps mature the eggs that are released at ovulation.

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. 

Multiple gestation pregnancies (pregnancies in which more than one fetus is carried) can also be due to the use of fertility drugs and in vitro fertilization (IVF), a process by which a fertilized egg (or multiple eggs) is implanted into the uterus. 

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.

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.

Are Twins Hereditary?

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, FSHB and SMAD3, 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.

Prevalence

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.

Contributing Hyperovulation Factors

Fertility drugs like Clomid (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

Taller people have higher levels of a protein called insulin-like growth factor (IGF). In people who ovulate, this protein can make the ovaries more sensitive to FSH, thereby increasing the chances of hyperovulation. 

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.

Identical twins originate from one egg that’s fertilized by one sperm cell. They share the same set of genetic material. The fertilized egg splits at some point shortly after fertilization. 

Those two developing eggs share a placenta (a structure that develops in your uterus, giving your baby oxygen and nutrients) in 70% of cases. They can also share an amniotic sac (the fluid-filled sac that surrounds and protects the fetus) or develop their own.

Fraternal twins develop from two different eggs, each fertilized by one sperm. They implant separately in the uterus and develop their own placentas and amniotic sacs.

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 have Down syndrome and the other not.

Fraternal Twin Types

Most fraternal twins arise from two eggs, released by the ovaries and fertilized by sperm at roughly the same time. But variations exist.

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

This is another rare occurrence. It happens when two eggs, released during the same menstrual cycle, are fertilized by sperm from two different people during two different episodes of sexual intercourse.

Testing, Growth, and Birth

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.

But the most definitive way to determine if the twins are fraternal or identical is to test the babies’ DNA after birth. This usually is done by swabbing the inside of the cheek. Fraternal twins will share roughly 50% of their genetic material.

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:

• Breathing 
• Feeding
• Staying warm
• Fighting infection

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.

While many sets of twins can—and are—delivered vaginally, the vast majority of twins—about 75% in the United States—are delivered by cesarean section (surgically). 

This may be because the twins are premature, one or the other baby is in a breech position (feet or buttocks instead of the head are pointed down toward the birth canal), health risks to the mom or baby, or even patient preference,

Parenting Fraternal Twins

Parenting your twins won’t necessarily be like parenting their siblings. Experts advise that you:

• Treat them as individuals: Make sure you recognize that each child will have unique personalities, talents, and emotions. 
• Separate your twins occasionally: You want each twin to learn how to develop and navigate relationships on their own. Spend one-on-one time with each; schedule separate play dates; enroll them in separate classes once they hit school age.
• Get help when you need it: Develop a childcare schedule with your partner and ask family and friends for help when you’re overwhelmed. Your pediatrician will also be an excellent source of information.

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 has schizophrenia (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

Fraternal twins—known medically as dizygotic twins—result when two separate eggs are fertilized by two separate sperm. The two separate eggs implant in the uterus and develop during one pregnancy.

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.

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