What is the meaning of fraternal twins?

Fraternal twins come from two separate fertilized eggs that develop in the womb at the same time, each with its own placenta and amniotic sac. Like any siblings, they may – or may not – share characteristics such as gender, hair color and eye color.

Unlike identical twins, which develop from the same egg that splits in two, fraternal twins do not share exactly the same genetic material. Because of this, twins do not look identical and can be boy/girl, not just girl/girl or boy/boy.

Most twins (70%) are cousins. Learn your chances of having twins.

How were fraternal twins conceived?

Fraternal twins come from two separate eggs fertilized by two separate sperm. In other words, more than one egg is released and fertilized in the same menstrual cycle.

The tendency to hyperovulate (release more than one egg during a cycle) can be inherited, which would explain why fraternal twins often run in families. But we don’t know for sure, and experts suggest other factors may be at play.

What increases the chances of having fraternal twins?

Your chances of having fraternal twins may be higher, depending on:

• Old age. Women over 35 are more likely to release more than one egg per menstrual cycle.
• Fertility treatments. The chances of having twins are increased with fertility drugs (which stimulate your ovaries to produce more eggs when you ovulate) and in vitro fertilization (IVF) if two embryos are placed in the uterus.
• Ethnicity. According to Centers for Disease Control and Prevention,Opens a new window Black women have more twins than white, Hispanic, or Asian women. The twin birth rate (births per 1,000 women) is 41 for black women, 33 for white women, 25 for Hispanic women, and 24 for Asian women.
• Family history. If there are twins in your family, the chances of having twins are higher.
• Number of pregnancies. Your chances of having twins increase with each pregnancy.
• Body type. Tall women have more twins than short women. StudiesOpens a new window attribute the link to insulin-like growth factor, which is associated with both height and twinning.

Differences between fraternal and identical twins

Here’s a guide to the main differences between the main types of twins – fraternal and identical.

fraternal twins:

• They come from two separate, fertilized eggs that develop in the womb at the same time
• Share some of their genes
• They can be different genders
• They have their own placenta and amniotic sac
• May or may not look similar (as with any other siblings)

identical twins:

• They develop from a single fertilized egg that splits into two parts
• You share the same DNA
• Are they the same gender?
• They may or may not share the placenta and amniotic sac. Most (70%) identical twins share a single placenta and have their own amniotic sac. Thirty percent each have their own placenta and amniotic sac. And rarely, identical twins will share an amniotic sac and placenta.
• They look almost exactly the same, although they may have some differences (for example, weight or birthmarks). Identical twins have different fingerprints and personality traits.

There is another, extremely rare type of twins called semi-identical (sesquizygotic) twins, who share more than half but less than all of their DNA. Semi-identical twins are formed when one egg is fertilized by two separate sperm and then splits into two separate embryos.

How do twins develop in the womb?

Fraternal twins develop in their own amniotic sac with their own placenta to provide oxygen and nourishment. So in a way, while they share the womb, they don’t have to share it everything.

“They’re like siblings living in the same house, but they each have their own room with a mini-fridge,” says Carolina Bibbo, MD, a maternal-fetal medicine specialist with a clinical focus on multiple pregnancies. (Dr. Bibbo is the director Multi-Person Comprehensive Care Center at Brigham and Women’s HospitalOpens a new window in Boston.) “These siblings tend to do really well!” she says.

Twins seem to enjoy each other’s company starting in the womb. One study found that twins interact with each other as early as 14 weeks pregnant, when they begin to reach toward each other.

As the pregnancy progresses, they deliberately spend more time reaching for their twin than for themselves or the walls of the womb. They even caress the head or the back of their sibling’s head more often than they touch each other!

Fraternal twins in the womb are like siblings who live in the same house, but each has their own room with a mini fridge.

– Carolina Bibbo, MD, maternal-fetal medicine specialist

There is no solid evidence that twins have a special emotional bond. But many parents believe that this early attachment continues after birth and throughout life. Gemini may find comfort in company, for example, and feel the need to be close to each other. Moms in the BabyCenter community Got Twins the group has different views:

“My twins/girls are 6 years old. They fight sometimes like all siblings. But 100% there is something special and amazing. They look out for each other.” – Loveskiing

“My children are 4 years old and they are very close. My son plays very well with boys and my daughter enjoys the quieter company of girls, but together they have their own style of play which is a mix of girls and boys. really balance each other out.” – seti1st

“My twins are 2 months old and couldn’t care less about each other. LOL.” – chrstnarn

Find out how your twins grow in the womb.

What to know if you are pregnant with twins

“Being pregnant with twins will likely mean you’ll have more frequent prenatal visits,” says Layan Alrahmani, MD, a board-certified gynecologist and member of BabyCenter’s medical advisory board.

“Your doctor will also want to do additional ultrasounds in your second and third trimesters to check on your babies. Some twin pregnancies become high-risk, in which case your doctor may want to refer you to a maternal-fetal medicine (MFM) physician to help you manage your pregnancy.”

“Don’t neglect your body,” advises BabyCenter community member and twin mom KBrink17. “If something doesn’t feel right, call your gynecologist, even if it’s just for your peace of mind. With gynecological exams and specialist appointments, you’ll feel like you’re living in a doctor’s office. But I found that it gave me a lot of comfort to know that we are so well taken care of.”

Carrying twins can mean you have extra pregnancy issues – more constipation, heartburn and morning sickness, for example.

“I never had morning sickness during my singleton pregnancy,” says BabyCenter community member SammyJuneLively. “That was my sign that they were twins.”

While most mothers of twins give birth to healthy babies, women pregnant with twins have an increased risk of:

• Premature birth. According to American College of Obstetricians and GynecologistsOpens a new window (ACOG), more than half of all twins are born prematurely.
• Intrauterine growth restriction (IUGR), which means the baby is not growing as expected
• Cerebral palsy, which is more likely in twin babies
• Amniotic fluid problems, such as too much amniotic fluid (polyhydramnios)
• Gestational hypertension
• Gestational diabetes
• Iron deficiency anemia. It is more than twice as common if you wear multiples.
• Preeclampsia. It develops more often in women carrying multiples, and according to ACOG, it also tends to occur earlier and be more severe.
• Cholestasis in pregnancy, a liver condition that causes severe itching
• Placental abruption, which means that the placenta has separated from the uterine wall before delivery
• A caesarean section, which is sometimes needed to deliver twins if the babies are in certain fetal positions or if labor is not progressing, for example.
• Postpartum bleeding, when the uterus has trouble returning to its normal size
• Postpartum depression

Although the potential risks may seem a little scary, know that most twin mothers have successful pregnancies and give birth to two healthy babies.

And if you’re wondering how you’ll manage when the twins are born, be sure to read our article on how to take care of newborn twins.