Types of Twins: Fraternal, Identical, and More

Finding out you’re having a baby is one of the most exciting times in your life. Finding out you’re expecting two babies is double the joy—and maybe twice as many questions, too! For example, you might be wondering about all the different types of twins, and what the difference is between fraternal and identical twins. Read on to find out all this and more, including how some of the different types of twins are formed. Keep in mind that we won’t cover all of the different types of twins in this article, but will look at some of the most common ones, such as fraternal twins and identical twins.

What Are Fraternal Twins (Dizygotic)?

The most common type of twins are called fraternal twins, also known as dizygotic twins. Below you can discover how they are formed and how similar these twins are.

How Are Fraternal Twins Formed?

Fraternal twins are formed as the result of two separate eggs being fertilized by two separate sperm. Two distinct embryos are then formed and each has its own placenta and amniotic sac. This can happen because the ovaries release two eggs and not just one.

How Similar Are Fraternal Twins?

Fraternal twins are as alike as siblings would be—in other words, they’re not identical. Fraternal twins might be two boys, two girls, or a boy and a girl. Each baby develops in their own placenta.

What Are Identical Twins (Monozygotic)?

Identical twins, or monozygotic twins, form when one fertilized egg splits in two and grows into two separate embryos. Identical twins may or may not share the same placenta but commonly have their own amniotic sac.

Can Identical Twins Be a Boy and a Girl?

It’s common to wonder if identical twins can be different genders. Identical twins are almost always the same sex at birth, share the same blood type, and share the same physical traits, such as eye and hair color.

Do Identical Twins Have the Same DNA?

Identical twins actually do have the same DNA at birth but eventually, the DNA becomes more distinctive based on environmental factors. This is how each twin evolves to be a unique individual. For more on your little one’s development, check out what happens week-by-week during a twin pregnancy.

Do Identical Twins Have the Same Fingerprints?

Although identical twins have the same DNA at birth, their fingerprints are actually different. No two fingerprints are exactly the same—making each of us unique!

What Are Conjoined Twins and How Do They Develop?

In some rare cases, the embryo only partially separates, and the two fetuses remain partially connected as they grow. Conjoined twins can connect at different spots, including the chest, abdomen, or hips. The twins may also share one or more internal organs. Conjoined twins are always identical. Sadly, many conjoined twins do not make it to full term or live only a short while after birth. However, advances in technology and surgery are increasing the chances of successful separation by surgery, depending on where the twins are conjoined and what organs they share.

Do Twins Share a Placenta and Amniotic Sac?

Sometimes twins share the placenta (the organ that connects mother to baby), amniotic sac (the inner membrane), and chorion (the outer membrane), and sometimes they each have their own. Your healthcare provider will be able to explain which of these is relevant for your pregnancy, and what it might mean for your pregnancy and babies.

• Diamniotic-dichorionic twins. These types of twins have their own chorions and amniotic sacs, but they may or may not share the same placenta.

• Diamniotic-monochorionic twins. These types of twins share a chorion and a placenta but are in separate amniotic sacs.

• Monoamniotic-monochorionic twins. These types of twins share a chorion, placenta, and an amniotic sac. This is the rarest type of twin, and it means a riskier pregnancy as the babies can get tangled in their own umbilical cords. If you have monoamniotic-monochorionic twins, your healthcare provider will monitor your pregnancy closely.

How Common Are Twins?

Multiple births—which include twins, triplets, quadruplets, and more—have become more common in recent years because more couples are turning to fertility drugs and IVF treatments to help them conceive. In the United States, twins account for about 3 percent of live births.

If you’ve just found out you’re pregnant with twins, read more about twin pregnancy and what to look forward to, and use our Due Date Calculator to get an estimate of when you might meet your little ones.

Can You Increase Your Chances of Having Twins?

Several factors can increase the chances of having twins, particularly fraternal (non-identical) twins. These factors may include:

1. Age. Women over the age of 35 have a higher chance of conceiving twins because they are more likely to release more than one egg during ovulation​

2. Family History. A family history of fraternal twins, especially on the mother's side, can increase the likelihood of twins.

3. Fertility Treatments. The use of fertility treatments, such as in vitro fertilization (IVF), greatly increases the chances of twins. Fertility drugs, which stimulate the ovaries to produce multiple eggs, can also result in multiple births​.

4. Body Mass Index (BMI). Those with a higher BMI (over 30) may have a higher chance of conceiving twins. This is thought to be related to higher levels of certain hormones​.

5. Previous Twin Pregnancy. If you've had twins before, you're more likely to conceive twins again​.

While these factors can increase your chances, there are no guaranteed methods to conceive twins naturally. Identical twins, on the other hand, occur randomly and aren't influenced by external factors​.

Do Twins Run in Families or Skip a Generation?

The idea that twins “skip a generation” is a common myth, but the reality is more nuanced. Whether or not twins run in families depends largely on the type of twins. Fraternal twins can run in families as the tendency to release multiple eggs (hyperovulation) may be inherited. This trait can be passed down through the maternal line, meaning if your mother or grandmother had fraternal twins, you might be more likely to have them too​. It doesn’t necessarily “skip” generations, but it may appear in various family members depending on the inherited trait.

Identical twins, on the other hand, are not influenced by heredity and happen randomly. There is no evidence that identical twins run in families​.

What Pregnancy Complications Are Associated with Twins?

Twin pregnancies are generally higher risk compared to single pregnancies, but most result in healthy outcomes with good prenatal care. Some common complications include:

• Preterm Birth. Twins are more likely to be born before 37 weeks. Some preterm twins may need to spend some time in the neonatal intensive care unit (NICU) before they’re ready to go home.

• Gestational Diabetes. Carrying twins increases your chance of developing gestational diabetes​.

• Preeclampsia. This is a condition marked by high blood pressure and other symptoms. It is more common in twin pregnancies and needs careful monitoring.

• Growth Restrictions. One or both twins may experience intrauterine growth restriction (IUGR), meaning they don’t grow as expected​.

With proper medical care and frequent checkups, most complications can be managed effectively​.

The Bottom Line

Although the biology behind twins can be tricky to completely understand, just know that having multiple babies is a miracle of nature. No matter what kind of twins you’re having, you’ll cherish those first few weeks with your twins, and the many milestones to come in the months and years beyond. Keep in mind that with twins you’ll be on double diaper duty, so why not earn rewards points for all those diapers? Download the Pampers Club app, and receive rewards for all your Pampers purchases.

_{How We Wrote This Article The information in this article is based on expert advice found in trusted medical and government sources, such as the American Academy of Pediatrics and the American College of Obstetricians and Gynecologists. You can find a full list of sources used for this article below. The content on this page should not replace professional medical advice. Always consult medical professionals for full diagnosis and treatment.}