What Are Dizygotic Twins?

Multiple births are becoming more common than they were in the past. Many women are waiting longer before having children than they did in previous generations. Fertility treatments and modern assisted reproductive techniques are resulting in more women having multiple children during pregnancy.

What Are Dizygotic Twins?

Twins of all kinds account for 90% of multiple births. There are many types of multiple births, but the most common type is dizygotic twins. Two-thirds of all sets of twins are dizygotic, while one-third are monozygotic.

Monozygotic twins are commonly known as identical twins. Dizygotic twins, the more common variety, are known as fraternal twins. These twins typically don’t look exactly alike and don't have to be the same gender, unlike monozygotic twins. Dizygotic twins are similar to other sets of siblings that aren’t twins, as they share half of their genes.

Differences Between Monozygotic and Dizygotic Twins

When two different eggs are fertilized during the same pregnancy, the result is a set of dizygotic twins. They are fertilized by two different sperm. Fraternal twins may look alike in many ways, but genetically they aren’t any different from a normal set of siblings.

Identical twins, or monozygotic twins, occur when one egg is fertilized by one sperm. Sometime during the first couple of weeks after conception, the egg splits in half, resulting in two, identical babies. These kinds of twins are always of the same gender, either two girls or two boys.

Dizygotic twins usually don’t share a placenta since they’re the result of two different eggs and sperm. They usually also develop two different amniotic sacs and support structures. However, identical twins may share an amniotic sac. Depending on when the egg split in half, these twins might or might not be in the same amniotic sac together.

How to Determine if Twins Are Identical or Fraternal

If the two babies are of different genders, then it’s quite easy to know that they are dizygotic twins. However, it might not be as easy to determine if the two babies are of the same gender. At the time of birth, doctors can take a look at the placenta and fetal membranes to see if the babies are monozygotic or dizygotic.

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Sometimes it’s possible to know if same-gender twins are identical or fraternal during pregnancy. If you have an ultrasound after 14 weeks of conception, your doctor or birth specialist might be able to determine what kind of placenta your babies have.

The best way to know if your set of twins is monozygotic or dizygotic is to have a DNA test done after they are born. Sometimes, determining the type of twins on the placenta alone can be unreliable. A family can be told that a set of twins is identical at birth based on the placenta alone, but as they grow older they start to notice more and more differences between the children. The opposite can also be true, that families are told that their twins are fraternal based on the placenta when really they are identical.

It’s important to note that even if a set of twins is identical, they might still have some physical differences. These differences can be caused by environmental factors, including positioning inside of the womb. Differences might develop after birth as a response to the babies’ environment. Pieces of their DNA can turn on or off depending on their surroundings, so twins may start to differ as they get older.

The Chances of Having Twins

The cause of monozygotic twins still isn’t known. All women have the same probability of conceiving identical twins, which is about 1 in 250. As of now, research suggests that there aren’t any genetic factors that increase the chance of having identical twins. However, some families have larger numbers of identical twins, which is currently being researched. Some researchers think there could be genes that stick cells together, which can be a factor in creating monozygotic twins.

When it comes to dizygotic twins, there are a few different known causes:

  • Certain ethnic groups report higher instances of fraternal twins
  • Running in families, especially on the mother’s side of the family
  • The mother’s body composition
  • The mother’s age, as older mothers have higher levels of estrogen and may produce more than one egg at a time
  • The use of assisted reproduction techniques, like in vitro fertilization (IVF)
  • The number of previous pregnancies the mother has had

What is known is that the frequency of dizygotic twins is on the rise in comparison to previous generations.

WebMD Medical Reference Reviewed by Dan Brennan, MD on March 02, 2021

Sources

SOURCES:

HealthyChildren.org: “The Difference Between Identical and Fraternal Twins.”

Medline Plus: “Is the probability of having twins determined by genetics?”

National Health Service: “Pregnant with twins.”

National Human Genome Research Institute: “Fraternal Twins.”

Twins Research Australia: “Types of twins.”

Victoria State Government BetterHealth: “Twins - identical and fraternal.”

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