What Parents Should Know About Fraternal Twins

Some parents are given a two-for-one deal when it comes to pregnancy with twins. Whether that sounds like a dream or a nightmare scenario for you, the fact is that twins are quite rare, accounting for only about 3% of live births in the U.S. Of those births, and excluding pregnancies that result from assisted reproductive technology, about 70% are fraternal twins (also known as dizygotic twins); the remainder are identical (monozygotic) twins.

Fraternal twins develop from the fertilization of two separate eggs with two different sperm. They share half of their genomes, like any other siblings, and they look completely distinct from one another. Unlike identical twins, they can also be different biological sexes.

Keep reading to learn more about fraternal twins, including common characteristics, risk factors, and what to expect during your pregnancy with two babies.

How Do Fraternal Twins Develop? 

When a person ovulates, a mature egg is released from their ovaries. It travels down the fallopian tube and awaits fertilization with sperm, which can result in conception. Eggs that don't get fertilized shed with the uterine lining during the menstrual period.

Sometimes a person's ovaries release two eggs instead of one. If both eggs are fertilized by separate sperm, you might end up with two embryos — also known as fraternal twins. Each embryo will implant separately in the womb with its own placenta and amniotic sac.

How Common Are Fraternal Twins?

The incidence of fraternal twins can vary based on a number of factors, including how the pregnancy was conceived, according to Sasha Andrews, M.D., a maternal-fetal medicine specialist at Pediatrix Medical Group. "Fraternal twins are more common in pregnancies conceived with assisted reproduction," says Dr. Andrews. This includes ovulation induction medications, intrauterine insemination (IUI), and in vitro fertilization (IVF).

Additionally, "as maternal age and/or the number of children increases, so does the chance of fraternal twins," says Dr. Andrews. The pregnant person's family history is another factor—if their mother or father has a history of twins, then they may be more likely to have twins as well.

One study found increased rates of twin pregnancies among those with taller height, higher body weight, and a history of smoking before conception. Another older study suggested that eating dairy products could increase the odds. The University of Rochester Medical Center says Black people assigned female at birth have a higher likelihood of twins, while Asian people and Native Americans have the lowest rates.

Geography can also raise the risk of fraternal twins, with some countries seeing a higher incidence than others. "The commonality of fraternal twins differs across different geographical regions," says registered nurse Shelly Patularu, a twin mom and founder of Twin Mom Guide, a resource for parents of twins. "Saudi Arabia, Japan, and Vietnam are said to have the lowest rates of twin births in the world. In comparison, Nigeria and Benin in West Africa have the highest twin birth rate." Other factors, such as diet, might affect the twin rates in these locations.

Fraternal Twin Pregnancies: What to Expect

Twin pregnancies tend to be more complex than typical pregnancies, according to Dr. Andrews. "Fraternal twins are more likely to have congenital abnormalities when compared to singletons, and also have a higher risk of fetal growth restriction, or small size at birth," she says. The risk of preterm birth is much higher as well; fraternal twins are often delivered around 36 weeks.

And the birthing parent is at higher risk, too. Maternal disorders such as preeclampsia and gestational diabetes can occur more frequently when carrying multiples. C-section delivery is also more common. But it's important to note that that most people have healthy pregnancies and babies.

Here are some complications associated with fraternal twin pregnancies, according to March of Dimes:

• Preterm labor or premature birth
• Gestational diabetes
• Preeclampsia
• Birth defects
• Anemia
• Low birth weight
• Postpartum hemorrhage
• Miscarriage
• Stillbirth

"Prenatal care for twin pregnancies is similar in some aspects, but given the increased risk of complications, more frequent ultrasounds and visits are usually recommended," says Dr. Andrews. "Patients with twins are often referred to a perinatologist, or high-risk pregnancy specialist, for additional counseling and monitoring."

Common Characteristics of Fraternal Twins

Unlike identical twins, fraternal twins will always have their own placenta and their own amniotic sac. They can also be different biological sexes, whereas identical twins will always share the same sex.

In fact, you can consider fraternal twins just like any set of siblings, says Patularu. "Because of this, fraternal twins may look similar or very different. They can be very different in size and development, with some fraternal twins reaching milestones at vastly different times."

One thing both fraternal and identical twins may have in common: they can create their own language. "Cryptophasia is the phenomenon of language developed by twins that only the two children can understand," Patularu says. "The secret language usually fades as their vocabulary grows and typically disappears by school age."

Are My Twins Identical or Fraternal?

Identical twins occur when one sperm fertilizes one egg, and the egg splits in two. They have the same genetics and biological sex, and they might share a placenta and/or amniotic sac. Still, it's not always easy to tell for sure whether twins are fraternal or identical after birth.

Sometimes health care providers can tell if twins are identical or fraternal through the placenta. If there's any doubt, parents may choose to do a twin zygosity DNA test in order to determine whether their twins are identical or fraternal. This is a simple test that can be performed after the babies are born, or later in life. The test consists of a simple, painless cheek swab collected from each twin, allowing a comparison of their DNA profiles. If they're an exact match, they're identical twins.