Twins happen in one of two ways: either a single fertilized egg splits into two embryos, or two separate eggs get fertilized by two different sperm at the same time. These two paths produce identical and fraternal twins, respectively, and they involve completely different biological mechanisms. About 31 out of every 1,000 births in the United States are twins, based on 2023 data from the CDC.
Identical vs. Fraternal: Two Different Processes
Fraternal twins (also called dizygotic twins) are the more common type. They form when a woman’s ovaries release two eggs during the same cycle instead of the usual one. Each egg is fertilized by a different sperm, creating two genetically distinct embryos. Fraternal twins share about 50% of their DNA, the same as any pair of siblings. They can be different sexes and often look no more alike than brothers and sisters born years apart.
Identical twins (monozygotic twins) start as a single egg fertilized by a single sperm. Within the first two weeks after conception, the developing embryo splits into two. Both halves carry the same DNA, which is why identical twins look so strikingly similar. Scientists still don’t fully understand what triggers this split. It appears to be a random event that isn’t influenced by family history, age, or lifestyle.
The timing of that split matters. When it happens later in embryonic development, the embryo already has a defined right and left side. The result is mirror-image twins, where features appear as reflections of each other. One twin may be right-handed while the other is left-handed, their hair whorls spiral in opposite directions, and even their first baby teeth may appear on opposite sides. About 25% of identical twins are mirror-image twins.
Why Some Women Are More Likely to Have Fraternal Twins
Since fraternal twins require two eggs, anything that increases the chance of releasing multiple eggs in a single cycle raises the odds. This process, called hyperovulation, is influenced by genetics, age, diet, and fertility treatments.
Genetics plays a meaningful role. Researchers have identified specific genes on chromosomes 1, 11, and 15 that are linked to a higher likelihood of fraternal twins, and there may be additional genetic factors not yet mapped. This is why fraternal twins often run in families, specifically on the mother’s side. The father’s genes don’t affect whether the mother releases multiple eggs, so a family history of twins on the father’s side doesn’t increase the odds in the same direct way.
Age is another factor. Women over 30 produce higher levels of the hormone that stimulates egg release, making multiple ovulations more likely. CDC data shows a clear pattern: the twin birth rate for women aged 30 to 39 is about 36 per 1,000 births, and for women 40 and older it climbs to 39 per 1,000.
How Diet May Play a Role
One of the more surprising influences on twinning rates is dairy consumption. A study published in The Journal of Reproductive Medicine found that vegan women, who exclude all dairy products, have a fraternal twinning rate just one-fifth that of vegetarians and omnivores. The connection appears to involve a growth-promoting protein found in milk. Blood levels of this protein rise in proportion to dairy intake, and higher levels can stimulate the ovaries to release more than one egg per cycle. In cows specifically bred for higher twinning rates, levels of this same protein are elevated.
This doesn’t mean drinking more milk guarantees twins. But the data suggests dairy consumption is one of several factors that can nudge the probability up or down.
Fertility Treatments and Twin Pregnancies
Fertility treatments are a major driver of twin pregnancies because they often involve stimulating the ovaries or transferring multiple embryos. The risk of multiples varies widely depending on the approach. According to the American Society for Reproductive Medicine, injectable hormones that stimulate ovulation carry the highest risk, with about a 30% chance (roughly 1 in 3) of a multiple pregnancy. Oral ovulation medications carry about an 8% chance (1 in 12). IVF with a single embryo transfer has the lowest risk at around 3% (1 in 33).
The trend in IVF has shifted strongly toward transferring just one embryo at a time, specifically to reduce twin and higher-order pregnancies. Twin pregnancies carry more health risks for both the mother and babies, so most fertility clinics now recommend single embryo transfer when possible.
What Happens When a Twin Disappears
Not every pregnancy that starts as twins ends that way. In vanishing twin syndrome, one embryo stops developing and is gradually reabsorbed by the body, the surviving twin, or the placenta. The pregnancy continues with a single baby. This is likely far more common than most people realize, but because it typically happens in the first trimester, many women never know a second embryo was present. It’s detected most often in IVF pregnancies, where early ultrasounds at six or seven weeks can spot two embryos before one disappears. Among IVF pregnancies involving multiples, estimates range from 7% to 36% for vanishing twin syndrome.
Shared Placentas and Unique Risks
How twins develop in the womb depends on their type and timing. Fraternal twins always have separate placentas and separate amniotic sacs because they implant independently. Identical twins may or may not share a placenta, depending on when the embryo split. Earlier splits (within the first few days) tend to produce separate placentas, while later splits result in a shared one.
Sharing a placenta creates a specific set of risks. The most significant is a condition where blood vessels in the shared placenta connect the twins’ circulatory systems unevenly. One twin receives too much blood flow while the other receives too little. This occurs in 10 to 15 percent of identical twin pregnancies that share a placenta. It requires close monitoring and, in serious cases, a procedure to correct the imbalanced blood flow. Twins with separate placentas don’t face this risk.
What Determines Whether You’ll Have Twins
For identical twins, there’s no known way to predict or influence the odds. The embryo split appears to be random.
For fraternal twins, the factors that increase your chances stack on top of each other: having a family history of twins on your mother’s side, being over 30, having had previous pregnancies, being taller, having a higher body mass index, consuming dairy products regularly, and undergoing fertility treatments. None of these guarantees twins, and many women with several of these factors never conceive multiples. But collectively, they explain why some families and some populations have noticeably higher twinning rates than others.