Ask any parent of twins, and they may tell you that taking two babies or toddlers out in public can sometimes feel like being a local celebrity! Whether your twins are riding together in a double stroller or walking side by side, people may smile, stare, or stop to ask questions.
If you’re expecting twins or already raising twins, congratulations! Twins—whether identical or fraternal—are two unique people. Each child has their own personality, interests, strengths, and needs.
In this article, we’ll answer some of the most common questions parents have about twins and take a closer look at a few common myths and misconceptions.
Fraternal Twins
Fraternal twins, also called dizygotic twins, form when two separate eggs are released during the same menstrual cycle and each egg is fertilized by a different sperm.
Genetically, fraternal twins are like any other brothers or sisters; they just happen to be conceived and born at the same time. They may look alike, look different, or be different sexes.
Because fraternal twins develop from two separate eggs from the very start, they usually have their own amniotic sacs and their own placentas, although placentas can sometimes grow close together and fuse.
Identical Twins
Identical twins, also called monozygotic twins, form when a single fertilized egg splits into two embryos early in pregnancy. How early this split happens determines whether the twins share a placenta or an amniotic sac.
Identical twins share the same DNA and are always the same sex. Even so, they are never exactly alike and may look or develop differently over time.
Are Twins Becoming More Common?
If it seems like twins are more common today than they were a generation or two ago, you’re right.
In the United States, twin birth rates rose steadily from the 1980s through the mid‑2010s and have since leveled off. In 1980, there were about 19 twin births for every 1,000 births. By the mid‑2010s, that number climbed into the low‑to‑mid 30s. The most recent data showed the twin birth rate at about 31 twins per 1,000 live births. While slightly lower than peak years, this rate remains much higher than it was several decades ago.
Main Reasons:
One reason is that more people are having children later in life. As a person gets older—particularly after age 30—the ovaries may be more likely to release more than one egg during a menstrual cycle. This can increase the chance of having fraternal twins, even without fertility treatment.
Another reason is the increased use of fertility treatments. Some treatments encourage the ovaries to release multiple eggs, while others may involve transferring more than one embryo. Although fertility specialists now work to reduce the chance of higher-order multiple pregnancies, such as triplets or more, twins are still more common than they were before these treatments became widely available.
Genders of Identical Twins
No. Identical twins cannot be one boy and one girl. Identical twins develop when one fertilized egg splits into two embryos, so they typically have the same sex chromosomes.
Boy-girl twins are fraternal twins. They develop when two separate eggs are fertilized by two different sperm—one carrying an X chromosome and the other carrying a Y chromosome. This results in one baby with XX chromosomes and the other with XY chromosomes.
Identical v. Fraternal?
Even identical twins are not necessarily exact copies of each other. They may have small differences, such as different hair patterns, fingerprints, or birthmarks. These differences can develop because of how the babies were positioned in the womb or because of experiences they have after birth.
Healthcare providers may try to determine whether same-sex twins are identical or fraternal by looking at ultrasound images during pregnancy or examining the placentas and membranes after delivery. However, these clues are not always definitive.
For example, identical twins can sometimes have two placentas, and two separate placentas can also grow together and appear to be one. Because of this, it is sometimes difficult to determine whether twins are identical or fraternal based on pregnancy or delivery findings alone.
Appearance can also be misleading. Identical twins may develop different features as they grow, while fraternal twins can sometimes look very much alike. When there is uncertainty, genetic testing may be the most reliable way to determine whether twins are identical.
Can identical twins grow to look different?
Identical twins begin with the same DNA, but their genes can become active in different ways as they grow. Differences in their environment and life experiences can influence how their genes are expressed, which may help explain why identical twins can become more different from one another over time.
The most reliable way to determine whether twins are identical or fraternal is through DNA testing. A simple cheek-swab test, often called a zygosity test, can accurately determine whether twins share identical genetic material.
Do twins run in the family?
It depends on the type of twins.
Fraternal twins can sometimes run in families, particularly through the mother’s side. Some people inherit a tendency for their ovaries to release more than one egg during a menstrual cycle, which increases the chance of having fraternal twins. This appears to be related to how the ovaries respond to reproductive hormones rather than simply having higher hormone levels.
Fertility treatments that stimulate ovulation can also increase the likelihood of fraternal twins.
Identical twins are different. Identical twinning occurs at a fairly consistent rate around the world and does not appear to be strongly influenced by family history.
Can they skip a generation?
Not exactly. The idea that twins always skip a generation is a common myth.
The tendency to release more than one egg during ovulation—which can increase the chance of fraternal twins—can be passed down through families. However, this inherited tendency only affects people who ovulate. This can sometimes make it appear as though twins “skipped” a generation when the trait was passed through someone who could not express it themselves.
Natural rates of fraternal twinning also vary among populations. The Yoruba people of Nigeria have one of the highest known natural twinning rates in the world, with estimates of more than 40 twin births per 1,000 births. Researchers believe this may be related to a combination of genetic and environmental factors.
Why are identical twins used in research?
Because identical twins share the same DNA, they give researchers a unique opportunity to study how genetics and life experiences influence health and development. By comparing identical twins, scientists can learn more about which traits are influenced by inherited genes and which may be shaped by a person’s environment and experiences.
One well-known example is the NASA Twin Study, which followed identical twins Mark and Scott Kelly. Scott spent nearly a year in space while Mark remained on Earth. By comparing the twins before, during, and after the mission, researchers were able to study how long-term space travel affected the human body.
Making sure your twins feel like individuals
Raising twins takes time, energy, and plenty of flexibility. One of the most important ways parents can support their twins’ healthy development is to remember that they are two separate individuals—not one unit.
Using each child’s name, noticing their unique personalities and interests, spending one-on-one time with each child, and celebrating their milestones in ways that reflect who they are can all help twins develop their own sense of identity.
The bond between twins can be incredibly special and is certainly something to celebrate. At the same time, each child deserves to be recognized, supported, and celebrated for the unique person they are.