Fraternal twins form when two separate eggs are fertilized by two separate sperm during the same menstrual cycle, resulting in two genetically distinct babies developing side by side in the uterus. Unlike identical twins, who split from a single fertilized egg and share all their DNA, fraternal twins share only about 50% of their genes on average, the same as any pair of siblings. They simply happen to share a womb and a birthday.
How Fraternal Twins Form
In a typical menstrual cycle, one ovary releases a single egg. Occasionally, the body releases two eggs at once, a process called hyperovulation. If both eggs are fertilized by different sperm cells, the result is fraternal (dizygotic) twins. Each embryo implants separately in the uterine wall, develops its own placenta, and grows inside its own amniotic sac. This is why doctors describe fraternal twin pregnancies as “dichorionic diamniotic,” meaning two placentas and two sacs.
Because each twin comes from a completely independent egg-and-sperm combination, fraternal twins can be the same sex or opposite sexes. They can look strikingly similar or no more alike than siblings born years apart. Data from large studies in the UK and US consistently show that about 57 to 60% of fraternal twin pairs are same-sex, while the remaining 40 to 43% are boy-girl pairs.
Why Some Families Have More Twins
The tendency to release multiple eggs per cycle runs in families, specifically through the mother’s side. A large genetic study published in the American Journal of Human Genetics identified two key gene variants that increase a woman’s chance of having fraternal twins. One variant raises levels of follicle-stimulating hormone (FSH), the hormone that triggers egg release. Women carrying one copy of this variant were 18% more likely to deliver twins; a second variant, working through a different pathway, increased the odds by 9% per copy. These genetic factors only matter in the person ovulating, which is why a family history of twins on the father’s side has no direct effect on the likelihood of fraternal twins.
Maternal age plays a significant role too. As women get older, hormonal shifts make hyperovulation more likely. The numbers are clear: twins account for about 2.2% of births to women ages 18 to 24, rising to 4.1% for women 30 to 34, and reaching 6.9% for women 40 and older. That roughly threefold increase between young adulthood and age 40 is one reason twin births have climbed over the past several decades, as more people delay childbearing.
Fertility Treatments and Twinning Rates
Fertility treatments are the other major driver. Medications that stimulate ovulation work by prompting the ovaries to release multiple eggs per cycle, which directly raises the chance of fraternal twins. Clomiphene citrate, one of the most commonly prescribed ovulation medications, has historically been linked to a multiple pregnancy rate of 8 to 10%, though more recent systematic reviews put the actual rate closer to 3.8%, with nearly all of those being twins rather than higher-order multiples. IVF also contributes when more than one embryo is transferred, though many clinics have shifted toward single-embryo transfers to reduce twin pregnancies.
Since the 1980s, the global twinning rate has jumped by a third, from about 9 twin deliveries per 1,000 to 12 per 1,000. That translates to roughly 1.6 million twin pairs born worldwide each year. Most of this increase reflects more fraternal twins, since identical twinning occurs at a steady rate of about 4 per 1,000 deliveries everywhere in the world regardless of age, genetics, or fertility treatment.
How Fraternal Twins Differ From Identical Twins
The core difference is genetic. Identical twins share 100% of their DNA because they originate from a single fertilized egg that splits early in development. Fraternal twins share an average of 50%, just like any two siblings from the same parents. This distinction matters in medical research: by comparing how often identical versus fraternal twins share a trait, scientists can estimate how much of that trait is driven by genetics versus environment.
During pregnancy, doctors can usually tell the two types apart with an early ultrasound, ideally in the first trimester. Fraternal twins each have their own placenta and amniotic sac, producing a characteristic “lambda sign” or “twin peak sign” on ultrasound, where the thick membrane between the two sacs creates a visible triangular wedge of tissue. Identical twins who share a placenta have a much thinner dividing membrane that appears as a simple “T sign.” This distinction is important because it determines how the pregnancy is monitored.
Pregnancy Risks With Fraternal Twins
Carrying fraternal twins comes with roughly double the complication risk of a singleton pregnancy. Each twin has its own full set of developing organs competing for nutrients and space in a single uterus, which puts extra strain on the mother’s body. Preterm birth is the most common concern. Twin pregnancies are significantly more likely to result in early delivery compared to singletons.
Preeclampsia, a dangerous rise in blood pressure during pregnancy, is also more frequent. Because the twins share the same uterine environment, a problem affecting one twin, such as growth restriction or infection, can create complications for the other. Most fraternal twin pregnancies are monitored more closely than singletons, with more frequent ultrasounds and checkups throughout the second and third trimesters.
Can Fraternal Twins Have Different Fathers?
In extremely rare cases, yes. A phenomenon called heteropaternal superfecundation occurs when two eggs released in the same cycle are fertilized by sperm from two different men during separate instances of intercourse within a short window, typically three to four days apart, though the gap can stretch up to 14 days. The result is fraternal twins who are genetically half-siblings rather than full siblings. Documented cases exist, though the phenomenon is rare enough that each confirmed case tends to appear as a published medical report.
What Fraternal Twins Share and Don’t Share
Fraternal twins can be any combination of sexes and blood types. They may share a strong family resemblance or look completely different from each other, since they inherit an independent random mix of genes from each parent. One twin might have curly hair and brown eyes while the other has straight hair and blue eyes. They can have different body types, different temperaments, and different susceptibilities to illness.
What they do share is a prenatal environment. They experience the same nutritional supply, the same maternal stress hormones, the same exposure to any substances the mother encounters during pregnancy. This shared environment is why fraternal twins often have more in common, in terms of birth weight, early development, and certain health outcomes, than ordinary siblings born years apart, even though their genetic overlap is the same 50%.