Twinning is the process by which a single pregnancy produces two offspring. It happens in one of two fundamental ways: either a single fertilized egg splits into two separate embryos, or two eggs are fertilized independently by two different sperm at roughly the same time. In the United States, the twin birth rate is about 31 per 1,000 live births, meaning roughly 1 in 33 deliveries results in twins.
Identical vs. Fraternal Twins
The two main categories of twinning differ in a basic biological way. Identical (monozygotic) twins form when one sperm fertilizes one egg, and that single fertilized egg then splits into two embryos. Because they originate from the same cell, identical twins share virtually the same DNA and are nearly always the same sex.
Fraternal (dizygotic) twins form when a woman releases two eggs during the same cycle and each egg is fertilized by a different sperm. Genetically, fraternal twins are no more alike than any other pair of siblings, sharing about 50% of their DNA. They can be the same sex or different sexes, and they often look no more similar than brothers or sisters born years apart.
When the Split Happens Matters
For identical twins, the timing of the embryo’s division determines how the pregnancy develops and what risks are involved. The key variable is how much of the surrounding structure has already formed by the time the split occurs.
- Days 1 to 3: The split happens before any of the pregnancy’s protective layers have developed. Each twin gets its own outer sac (chorion) and its own inner sac (amnion). This setup closely resembles a fraternal twin pregnancy and carries the lowest risk.
- Days 4 to 8: The outer sac has already formed, so the twins share it. However, each twin still develops inside its own inner sac. This is the most common arrangement for identical twins.
- Days 8 to 13: Both the outer and inner sacs have formed, so the twins share a single enclosed space with no membrane between them. This is the highest-risk configuration because the babies can compress each other’s umbilical cords.
- Days 13 to 17: The embryo itself has already started developing by this point. A split this late is incomplete, resulting in conjoined twins, an extremely rare outcome.
What Increases the Chance of Twins
Most of the known risk factors apply to fraternal twinning specifically, because they relate to the release of multiple eggs.
Maternal age is one of the strongest predictors. As a woman’s ovarian reserve naturally declines with age, her body compensates by producing more follicle-stimulating hormone (FSH). This hormonal “overshoot” can trigger the development of multiple egg-containing follicles in a single cycle, increasing the odds that two eggs will be released and fertilized.
Genetics play a measurable role. A large study that included roughly 90,000 births in Iceland identified two specific genetic variants linked to fraternal twinning. One variant, located near a gene that controls FSH production, increased a mother’s chance of delivering twins by 18% per copy of the gene she carried. A second variant, involved in how the ovaries respond to FSH, raised the odds by 9% per copy. This is why fraternal twins tend to run in families on the mother’s side.
Fertility treatments significantly raise twinning rates. When two embryos are transferred during IVF, about 25% of resulting pregnancies are twins. The shift toward transferring a single embryo has reduced this, though single-embryo transfer does come with a lower per-cycle success rate.
Vanishing Twin Syndrome
Many more pregnancies begin as twins than end that way. Vanishing twin syndrome occurs when one twin stops developing early in pregnancy and is gradually reabsorbed. Estimates suggest this happens in 15% to 36% of twin pregnancies. With the widespread use of early ultrasound, it’s now detected more frequently than in past decades, when many women never knew a second embryo had been present. In most cases, the remaining twin continues to develop normally, and the pregnancy proceeds as a singleton.
Complications Unique to Identical Twins
Because identical twins who share an outer sac also share a single placenta, they face a complication that fraternal twins do not. Twin-to-twin transfusion syndrome (TTTS) occurs when blood flow through the shared placenta becomes unbalanced, with one twin receiving too much blood and the other too little. It affects roughly 8 to 10% of identical twin pregnancies where the babies share a placenta but have separate inner sacs. The condition is monitored through regular ultrasound, and when caught early, interventions can significantly improve outcomes for both babies.
A Rare Third Category
In extremely rare cases, twins are neither fully identical nor fully fraternal. These “semi-identical” (sesquizygotic) twins are thought to form when a single egg is fertilized by two sperm simultaneously. The resulting cell contains three sets of chromosomes instead of the normal two, and during its first division, it sorts those chromosomes into two cell lines that go on to become separate embryos. The twins end up sharing all of their mother’s DNA but only a portion of their father’s, placing them genetically somewhere between identical and fraternal. Only a handful of confirmed cases exist in the medical literature, making this the rarest known form of human twinning.