Identical twins form when a single fertilized egg splits into two separate embryos, producing two babies with the same DNA. They account for about 4 out of every 1,000 deliveries worldwide, a rate that stays remarkably consistent across populations and ethnicities. Fraternal twins, by contrast, come from two separate eggs fertilized by two separate sperm, making them no more genetically alike than any pair of siblings.
How a Single Egg Becomes Two Babies
After a sperm fertilizes an egg, the resulting cell (called a zygote) begins dividing. Normally this process produces one embryo. In identical twinning, something disrupts the normal path, and the growing cluster of cells separates into two distinct groups, each capable of developing into a full baby.
The split most commonly happens at the blastocyst stage, about four to eight days after fertilization. At this point, the embryo is a hollow ball of cells with an inner clump that will become the baby. If the cells in that inner clump are loosely connected, fluid accumulating inside the embryo can physically push them apart into two separate groups. In other cases, the embryo begins hatching out of its protective shell through a narrow opening, and the pressure of squeezing through that gap divides the inner cell mass in two.
Scientists still don’t fully understand why the split happens. It doesn’t appear to run in families the way fraternal twinning does. Research on pregnancies conceived through fertility treatments suggests that higher maternal age slightly increases the odds, and that transferring embryos at the blastocyst stage (day five) rather than earlier stages raises the rate of identical twinning as well.
When the Split Happens Changes Everything
The timing of the split determines how the twins develop in the womb and what risks they face during pregnancy. Each twin needs a placenta (which delivers nutrients from the mother) and an amniotic sac (the fluid-filled membrane surrounding the baby). Whether the twins get their own or share depends entirely on how many days after fertilization the division occurs.
- Days 1 to 4 (before the blastocyst forms): Each twin gets a separate placenta and a separate amniotic sac. These are called “di-di” twins. They carry the lowest risk of complications, and on ultrasound they look indistinguishable from fraternal twins.
- Days 4 to 8: The twins share one placenta but each has a separate amniotic sac. These “mono-di” twins are the most common type of identical twin pregnancy. Sharing a placenta introduces the possibility of unequal blood flow between the two babies.
- Days 8 to 12: The twins share both a placenta and a single amniotic sac. These “mono-mono” twins are rare and carry additional risk because the babies can become entangled in each other’s umbilical cords.
- After day 12: The split is incomplete, resulting in conjoined twins. This is extremely rare.
Shared Placenta, Shared Risks
About two-thirds of identical twin pregnancies are monochorionic, meaning the twins share a single placenta. Blood vessels inside that shared placenta can create connections between the two babies’ circulatory systems. When blood flows unevenly through those connections, one twin (the “recipient”) gets too much blood while the other (the “donor”) gets too little. This condition, called twin-to-twin transfusion syndrome, affects roughly 8% to 12% of mono-di pregnancies.
TTTS is serious. Without treatment, survival of at least one twin ranges from 15% to 70%, with both twins surviving about 50% of the time. The recipient twin’s heart can become overloaded, and the donor twin can become growth-restricted. TTTS also raises the risk of premature delivery and neurological complications. Pregnancies with a shared placenta are monitored with ultrasounds every two weeks starting in the second trimester to catch these problems early.
Not Quite 100% Identical
Identical twins start with the same DNA sequence, but they are not perfect genetic copies for their entire lives. Small random mutations can occur in one twin and not the other each time their cells divide. More significantly, the way their genes are read and used changes over time through a process called epigenetic modification, where chemical tags on DNA turn genes on or off without altering the underlying code.
A landmark study published in PNAS found that young identical twins are epigenetically indistinguishable, but older pairs show striking differences. Among the twin pairs studied, 35% had significant differences in these chemical markers. The pattern was clear: the youngest pairs looked nearly identical at the molecular level, while the oldest pairs had drifted far apart. Twins who spent more of their lives apart or had very different lifestyles showed the greatest divergence. This “epigenetic drift” helps explain why identical twins can develop different diseases, different body compositions, and even slightly different physical features as they age.
Mirror-Image Twins
About 25% of identical twins display a curious phenomenon called mirror imaging. One twin may be left-handed while the other is right-handed. Birthmarks, cowlicks, or even dental patterns appear on opposite sides. In rare cases, internal organs can be reversed in one twin.
This likely happens because the split occurs after the embryo has already begun establishing its left-right body plan. When the cell mass divides, each half carries a mirrored version of that plan. The later the split, the more pronounced the mirror effect tends to be.
How Twins Are Confirmed as Identical
You can’t always tell whether twins are identical just by looking at them. Even during pregnancy, ultrasound has limitations. When performed in the late first or early second trimester, up to 19% of high-risk monochorionic twins are incorrectly classified as having separate placentas. And di-di identical twins look exactly like fraternal twins on imaging.
DNA testing is the definitive method. Modern tests analyze thousands of genetic markers called single-nucleotide polymorphisms (SNPs), essentially comparing the twins’ DNA at thousands of specific locations across the genome. When identical, those markers match perfectly. A 2019 validation study found this approach correctly identified zygosity in 100% of cases, examining up to 12,568 markers per test. This kind of testing can now be done during pregnancy through a blood draw from the mother or after birth with a simple cheek swab.
Semi-Identical Twins
In extraordinarily rare cases, twins fall somewhere between identical and fraternal. Called sesquizygotic or “semi-identical” twins, they result from two sperm fertilizing a single egg at the same moment. The resulting cell mass contains three sets of chromosomes instead of the normal two, and it divides into two embryos that each keep two sets. These twins share all of their mother’s DNA but only a portion of their father’s. Only two confirmed cases have ever been recorded in the medical literature, including a boy-girl pair identified in 2019.