Facts About Identical Twins

While they may look alike, identical twins are unique individuals with their own personalities and interests.

Identical twins, also known as monozygotic twins, share the same egg at conception and are always either both boys or both girls. Fraternal twins, also known as dizygotic twins, come from two separate eggs fertilized at the same time. They could be the same sex or different. 

Factors that increase the chances of having twins include the use of fertility drugs or treatments, a maternal age of over 30 years old, and a family history of twins or multiples. Twins and multiple pregnancy babies are at increased risk of being born prematurely and with low birth weight. 

This article will discuss common misconceptions about identical twins. It will also describe their similarities and differences, as well as tips for parenting twins. 

Twin brothers hugging and standing in an urban scene


Types of Twins

The two types of twins are identical and fraternal. They differ in the way they are conceived and their genetic makeup. 

Fraternal vs. Identical 

Identical twins share 100% of their genes. They are always born of the same sex. It can be challenging to tell identical twins apart. Whereas fraternal twins often look more like siblings than twins. They share 50% of their genes and could be of the same or different sexes at birth. 


The differences between identical and fraternal twins are due to how they are conceived. When identical twins are conceived, the fertilized egg splits into two, causing two separate embryos to grow. When fraternal twins are conceived, two eggs are fertilized at the same time. 

Factors that increase your chances of having twins include:

  • Family history of multiple pregnancies
  • Maternal age of over 30
  • History of past pregnancies
  • Use of reproductive technologies like ovulation-stimulating medicines and in vitro fertilization (IVF)

Third Twin Theory 

There is a theory that there is a third type of twin in between identical and fraternal. Semi-identical twins are a rare type of twins who share 100% of the genes from their mother but only 50% to 75% of the genes from their father. How is this possible? Well, the theory is that when two separate sperm fertilize the same egg and the egg splits into two embryos, you get semi-identical twins. 

Role of Twin Genes in Family History 

You may have heard that twins run in families. However, experts believe that conceiving twins is affected by genetic and environmental factors. Identical twins make up about 3% to 4% of live births in the United States. The cause of having identical twins is usually unknown.

Fraternal twins, on the other hand, may run in families. Fraternal twins make up about 6% to 8% of live births in the United States. A woman who has a mother or sister who had fraternal twins is twice as likely to have them herself. 

Fraternal twins happen when a woman releases multiple eggs (instead of one) during the menstrual cycle, which is known as hyperovulation, and may run in families. Factors that increase your risk of hyperovulation include:

  • Maternal age
  • Race
  • Ethnicity
  • Body composition
  • Number of previous pregnancies
  • Use of reproductive technology

Benefits of Identical Twin Studies

Identical twins are incredibly helpful to researchers when they choose to participate in twin studies. Health researchers often examine the influence of both our genetic makeups and our environments to understand the causes of health conditions. This is often referred to as “nature or nurture.” 

Because identical twins share 100% of their genes, we can assume that any differences between the two of them are related to environmental factors. Twin studies in the past have examined up to 18,000 human traits such as height, weight, and chronic diseases to determine how influenced they are by our genes or our environment. 

Identical Twin Statistics

The identical twin birth rate in the United States is 32.1 per 1,000 live births. In 2019, about 120,291 sets of twins were born.

Testing, Growth, and Birth

A twin pregnancy is typically more complicated than a singleton pregnancy (with one baby). Twin pregnancies require more frequent prenatal health visits and tests. 

The first prenatal test to consider is diagnosing a twin pregnancy. Research suggests that up to 40% of twin pregnancies are not diagnosed until the 13th week of pregnancy. An ultrasound can determine if twin babies are identical or fraternal. 

Twin pregnancies are also more likely to end in preterm delivery. Preterm delivery occurs before 37 weeks gestation. Full gestation is 40 weeks. One of the reasons why twins are more likely to be born early is because the usual medical interventions that prevent preterm labor in singleton pregnancies are not effective for twin pregnancies. Because twins are more likely to be born early, they are also more likely to be considered low-birth-weight.

Labor and delivery are different and more complicated for twin pregnancies as well. For a twin pregnancy, the mother is usually taken to the operating room instead of a standard delivery room. This is because twin pregnancies are more likely to require cesarean section (C-section) surgery. However, it is possible for mothers of twins to deliver vaginally. Talk with your healthcare provider about how to prepare. 

Parenting Identical Twins 

Parenting identical twins comes with its own special challenges and rewards. It is helpful to prepare for twins’ unique parenting needs before they arrive and as they grow. 

Special considerations when raising twins:

  • Feeding challenges: Because twins are more likely to be born prematurely, they have serious nutritional needs as newborns, and it can be challenging to keep up with feeding both babies. 
  • Financial pressures: Parents of twins often need to buy double the diapers, clothing, formula, baby food, and equipment as parents of single babies. 
  • Social needs: While twins may look alike, they are different people with unique personalities. Twins are often grouped together but benefit when adults treat them as individuals and encourage them to pursue their interests separately. 
  • Sibling jealousy: Twins can be quite time-consuming, and older siblings may feel left out. Try to include your other children in the care of the twins and make some special time for them. 

How parents and caregivers can support their twins:

  • Plan alone time: Parenting twins is demanding, and it can be difficult to spend alone time with each child. As often as you are able, spend time with one twin at a time to give them the attention they crave. 
  • Encourage individualism: Give your twins a chance to develop their own personalities outside of one another by encouraging them to pursue their own interests and play with their own friends. This may also help to reduce the chance that your twins become overly competitive with or interdependent on each other. 
  • Make a school plan: Once your twins are old enough for school, work with their teachers to determine if they should be in the same class or separate classes. 

Parenting twins can be both incredibly rewarding and exhausting. It is important to keep your own needs in mind, such as:

  • Socialization: It is often more difficult for twin parents to get out of the house and see their friends. Seek support where you can. 
  • Mental health support: Postpartum depression (PPD) is more common in women who have had twins. Know the signs of PPD and talk with your healthcare provider if you are concerned.
  • Financial resources: It’s estimated that the healthcare delivery cost for a twin birth is four times higher than for singleton births. Check with your insurance company and hospital billing department to understand your out-of-pocket costs. 
  • Rest: Caring for twins is physically demanding and emotionally stressful. As often as you can, take time to rest and catch up on sleep. 

How to Tell Your Twins Apart

Even parents can have a difficult time telling their identical twins apart. To tell newborn twins apart, try color-coding their clothing or using name bracelets. As they age, their unique personalities will tell them apart. 


Identical twins have fascinated researchers and parents for decades. When they are conceived, a single fertilized egg splits into two embryos. While identical twins look alike, their personalities, interests, and needs can be very different. 

A Word From Verywell 

If you are a parent to twins, you know how much joy they can bring to your life. It’s also helpful to name the unique challenges so that you can seek out the support you need. When your twins are babies, ask for help with childcare, cooking meals, house cleaning, and other tasks. 

Frequently Asked Questions

  • Do identical twins read each other’s minds?

    Identical twins have unique ways of communicating with one another. While they cannot actually read each other’s minds, they learn each other’s facial expressions and gestures because they spend so much time together. This gives them unique insight into each other. 

  • Are identical twins rare?

    Identical twins make up about 3% of live births in the United States. 

  • Which parent passes on the twin gene?

    Fraternal twins tend to run in families. This may be due to hyperovulation, releasing more than one egg during the menstrual cycle, which is genetic. Because the mother is the one who ovulates, she is the one who is responsible for the increased chance of twins.

  • What are other ways to tell identical twins apart?

    Identical twins can be especially difficult to tell apart as newborns, even for their parents. If you are concerned about telling your babies apart, consider color-coding their clothing or having them wear bracelets with their names on them. As they age, their unique personalities will help to distinguish them from one another.

13 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Carrie Madormo, RN, MPH
Carrie Madormo, RN, MPH, is a health writer with over a decade of experience working as a registered nurse. She has practiced in a variety of settings including pediatrics, oncology, chronic pain, and public health.