Fraternal twins are conceived when two separate eggs are released during the same menstrual cycle and each is fertilized by its own sperm cell. Unlike identical twins, which form from a single fertilized egg that splits, fraternal twins start as two completely independent embryos. This means they share about 50% of their DNA, the same as any siblings born years apart.
How Two Eggs Get Released at Once
In a typical cycle, several egg-containing follicles begin developing in the ovaries, but only one becomes dominant and releases a mature egg. The process is controlled by follicle-stimulating hormone (FSH), which triggers follicle growth early in the cycle. Normally, once a dominant follicle takes over, hormone feedback signals the brain to dial back FSH production, preventing other follicles from maturing.
Fraternal twins become possible when this feedback system doesn’t fully suppress the other follicles. Extra FSH allows two (or more) follicles to reach maturity and release eggs, a process called hyperovulation. Each egg then travels down its own fallopian tube, where it can be fertilized independently. Because two separate sperm cells are involved, the resulting embryos can be different sexes and look no more alike than ordinary siblings.
Why Some Women Are More Likely to Hyperovulate
Age
Women in their mid-to-late 30s have the highest natural rate of fraternal twinning. As the ovarian reserve shrinks with age, the body compensates by producing more FSH to push follicles along. Research comparing women with single versus multiple follicle development found that those growing more than one large follicle had a mean age of 36.1 years and significantly higher baseline FSH levels (10.3 versus 7.7 IU/l) compared to women developing just one. In effect, the body overshoots its hormonal signal, and two eggs mature instead of one.
Genetics
The tendency to release multiple eggs runs in families, specifically on the mother’s side. Two genes involved in ovarian signaling, GDF9 and BMP15, play a key role. A study of mothers who conceived fraternal twins found that 4.12% carried rare variants in GDF9, compared to just 2.29% of women in a control group. These genetic differences appear to alter how follicles respond to growth signals, making double ovulation more likely. If your mother or maternal grandmother had fraternal twins, your own odds are elevated for this reason.
Body Size
Taller and heavier women conceive fraternal twins at higher rates. Women with a BMI over 25 have roughly 1.3 to 1.4 times the odds of having fraternal twins compared to women at a lower BMI. Women 5 feet 8 inches (173 cm) or taller show a similar increase. One explanation is that a growth-promoting protein called insulin-like growth factor (IGF), which circulates at higher levels in larger individuals, may encourage extra follicles to develop and survive.
Hormonal Shifts
Certain life transitions can temporarily raise the chance of hyperovulation. Teenagers, whose hormonal regulation is still maturing, sometimes release multiple eggs because their feedback systems aren’t yet fine-tuned. On the other end, women who recently stop using hormonal birth control may experience a brief surge in FSH as their bodies recalibrate, creating a short window of increased hyperovulation risk.
Fraternal Twins Through Fertility Treatment
Fertility treatments are a major driver of fraternal twin pregnancies because they deliberately stimulate the ovaries. The risk varies widely depending on the approach. With IVF using a single embryo transfer, the chance of multiples is about 3%. Medications like clomiphene citrate, which prompt the ovaries to develop more follicles, carry roughly an 8% chance. Injectable hormones called gonadotropins push that figure to around 30%, or about 1 in 3 pregnancies.
In each of these scenarios, the underlying mechanism is the same: more than one egg becomes available for fertilization. With IVF, twins can also result from transferring two embryos, each of which implants separately, producing fraternal twins even if the eggs were fertilized in a lab rather than in the body.
What Happens After Fertilization
Once both eggs are fertilized, each embryo implants independently in the uterine wall and develops its own placenta, its own blood supply, and its own amniotic sac. This arrangement is called dichorionic diamniotic, and it’s the standard configuration for fraternal twins. Doctors can identify it on ultrasound as early as 10 to 14 weeks by looking for a characteristic triangular wedge of tissue where the membranes separating the twins meet the uterine wall.
This separate-everything setup is one reason fraternal twin pregnancies tend to carry fewer complications than identical twin pregnancies. Because each baby has a dedicated placenta, there’s no risk of unequal blood sharing between twins (twin-to-twin transfusion syndrome), selective growth restriction from a shared placenta, or cord entanglement. Fraternal twins still count as a higher-risk pregnancy compared to a singleton, but the risks specific to shared placental structures don’t apply.
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 different men during separate acts of intercourse within a short time window. Because sperm can survive in the reproductive tract for up to five days and the two eggs are available around the same time, fertilization by two different partners is biologically possible. Documented cases exist, confirmed through paternity testing, but the occurrence is vanishingly rare in the general population.
Fraternal Versus Identical: The Core Difference
The distinction comes down to a single event. Identical twins start as one fertilized egg that splits into two embryos, producing genetic copies. Fraternal twins start as two eggs fertilized by two sperm, producing genetic siblings who simply happen to share a womb. Fraternal twins can be any combination of sexes, can have different blood types, and often look no more similar than brothers and sisters born years apart. About two-thirds of all twin pregnancies are fraternal.