By: Shelly Vaziri Flais, MD, FAAP
Ask any parent of twins: if you've ever wanted to feel like a celebrity, try taking a pair of twins to the grocery store. Whether your twins are babies riding in a double stroller or toddlers walking side by side, strangers will often smile, stare or stop to ask questions.
If you're expecting twins or raising young twins, congratulations! Twins—whether identical or fraternal—are
two separate, unique individuals, each with their own personalities, strengths and needs.
Below are answers to some of the most common questions parents ask about twins, along with explanations that clear up common myths.
How are fraternal twins conceived?
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.
How are identical twins formed?
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 twin births more common than they used to be?
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.
Researchers point to two main reasons for this long‑term rise in twin births.
One reason is that more people are having children later in life. After age 30, ovaries are more likely to release more than one egg during a menstrual cycle, which increases the chance of fraternal twins even without fertility treatment.
The second reason is the increased use of fertility treatments. Some infertility treatments stimulate ovulation or involve transferring more than one embryo. Although modern fertility care aims to reduce higher‑number multiples such as triplets, twins remain more common than they were before these treatments became widely used.
Can identical twins be boy and girl?
No. Identical twins cannot be boy and girl. Girls have two X chromosomes, while boys have one X and one Y chromosome. Eggs from the mother always carry an X chromosome, and the sperm from the father determines the baby's sex. Boy‑girl twins occur when two separate eggs are fertilized—one by an X‑carrying sperm and the other by a Y‑carrying sperm—which means they are fraternal twins.
How can you tell if twins are identical or fraternal?
Even identical twins often have small differences, such as cowlicks in the hairline, fingerprints or birthmarks. These differences can result from their position in the womb or from experiences after birth.
Health care professionals may try to estimate whether same‑sex twins are fraternal or identical based on ultrasound findings during pregnancy, or by examining the placentas and membranes at delivery. While this information can be helpful, it is not always exact. Some identical twins have two placentas, and placentas can fuse. When this happens, it can be hard to tell the difference.
Because of this, families are sometimes told that their twins are fraternal when they are actually identical. In other cases, parents may assume twins are fraternal because of differences in appearance, even though identical twins can look less alike over time.
Can identical twins become more different as they grow?
Although identical twins start out with the same DNA, how their genes are expressed can change in response to the environment. This helps explain why identical twins may become more different as they grow.
The only way to know for sure whether twins are identical or fraternal is through DNA testing. Simple cheek‑swab "zygosity" tests are widely available and can accurately determine this.
Do twins run in families?
Whether twins run in families depends on the type of twin. Fraternal twins can run in families through the mother. Some people inherit a tendency for the ovaries to release more than one egg during a single cycle, which increases the chance of fraternal twins. This tendency appears to be related to how the ovaries respond to reproductive hormones, not simply producing higher hormone levels.
Fertility treatments that stimulate ovulation can also increase the chance of fraternal twins. In contrast, identical twinning occurs at a fairly constant rate worldwide and does not appear to run in families.
Is it true that twins skip a generation?
The common belief that twins "skip a generation" is a myth. A genetic tendency for fraternal twinning can be passed down through families, but only shows up in people who ovulate which can make it seem as though twins skip generations.
Some populations have higher natural fraternal twinning rates. For example, the Yoruba people of Nigeria have the highest known rates in the world, estimated at over 40 twin births per 1,000 births, likely due to a combination of genetic and environmental factors.
Why are identical twins used in research studies?
Because identical twins share the same DNA, researchers can study how genes and environment each influence health and development. Comparing identical twins helps scientists better understand the balance between inherited traits and life 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, allowing researchers to study how space travel affects the human body.
How can parents support twins as individuals?
Raising infant twins is an investment of time and work, and parents can support their twins' healthy development by remembering that twins are two distinct, individuals. Using each child's name, recognizing differences in personality and interests, spending one‑on‑one time with each child, and celebrating milestones in ways that reflect each child's individuality can all help.
The bond between twins is special and worth celebrating—but so is each child's unique identity.
About Dr. Flais
 Shelly Vaziri Flais, MD, FAAP, is a board-certified practicing pediatrician and mother of 4 children. An assistant professor of clinical pediatrics with Northwestern University Feinberg School of Medicine and Ann & Robert H. Lurie Children's Hospital of Chicago, she is the author of the American Academy of Pediatrics books Raising Twins, and
Nurturing Boys to Be Better Men: Gender Equality Starts at Home. Dr. Flais is also editor-in-chief of
Caring for Your School-Age Child, 3rd Edition, and a contributor for
Sleep: What Every Parent Needs to Know. She has shared her reality-based parenting approach with national and local television, radio, online, and print news outlets.
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