Dizygotic twins

Medically Reviewed by Dr. Silvana Chedid on July 31, 2026

Dizygotic twins, more commonly called fraternal twins, occur when a woman releases two eggs in the same cycle and each egg is fertilized by a different sperm, producing two genetically distinct embryos. Because they come from two separate fertilized eggs rather than one, dizygotic twins can be different sexes and generally look no more alike than any other two siblings.

At Chedid Grieco, questions about dizygotic twins come up often, both from patients who conceived twins naturally and from those exploring fertility treatments where a twin pregnancy is a possibility. Understanding how dizygotic twins form, and how they differ from identical twins, helps set accurate expectations either way.

How Dizygotic Twins Form

In a typical cycle, one ovary releases a single mature egg. Occasionally, though, two eggs are released and both are fertilized, each by its own sperm cell. The two fertilized eggs implant separately in the endometrium, the inner lining of the uterus, and develop into two individuals with distinct genetic material, including sex chromosomes, which is why dizygotic twins can be a boy and a girl, two boys, or two girls.

Each dizygotic twin typically develops its own gestational sac and placenta, although the two placentas can occasionally fuse together as the pregnancy progresses. Because the twins are developing separately from the start, a dizygotic twin pregnancy can usually be diagnosed by ultrasound as early as six weeks after fertilization.

Dizygotic Twins vs. Identical Twins

The key difference between dizygotic and identical, or monozygotic, twins comes down to how many eggs and sperm were involved. Identical twins develop when a single fertilized egg splits into two embryos early in development, which means they share virtually all of their genetic material and are always the same sex. Dizygotic twins develop from two separate eggs and two separate sperm, so they share roughly half their genes, the same genetic overlap as any full siblings, just conceived at the same time.

According to the National Human Genome Research Institute, fraternal twins make up the clear majority of twin pregnancies, accounting for roughly two-thirds of all twin births, compared with about one-third that are identical.

What Increases the Chance of Dizygotic Twins

Several factors are associated with a higher likelihood of releasing more than one egg in a cycle, which is the underlying event that leads to dizygotic twinning.

• Family history: A history of fraternal twins on the mother’s side can increase the likelihood of hyperovulation, the release of more than one egg in a cycle.
• Maternal age: Women in their late thirties may release more than one egg per cycle more often than younger women, due to hormonal shifts that occur as ovarian reserve changes.
• Fertility treatment: Treatments that stimulate the ovaries, such as ovulation induction medications or IVF protocols involving multiple embryo transfer, can increase the chance of a multiple pregnancy, including dizygotic twins.

Patients undergoing fertility treatment who want to understand their personal likelihood of a twin pregnancy should discuss their specific protocol and embryo transfer plan directly with their fertility team, since the risk varies considerably by treatment type.

Monitoring a Dizygotic Twin Pregnancy

Twin pregnancies, dizygotic or identical, are generally monitored more closely than single pregnancies because of the higher likelihood of complications such as preterm labor, gestational diabetes, and blood pressure disorders. Regular ultrasounds track each twin’s individual growth, since dizygotic twins with separate placentas typically grow independently of one another rather than sharing a single blood supply.

Because dizygotic twins each have their own placenta from the start, some of the complications specific to identical twins who share a single placenta, such as twin-to-twin transfusion syndrome, do not apply in the same way. Your obstetric team will outline a monitoring schedule specific to your pregnancy once twins are confirmed.

Frequently Asked Questions About Dizygotic Twins

Whether you are curious about a twin pregnancy you are already expecting or you are exploring fertility treatment that could increase the likelihood of twins, our team can walk you through what to expect at every stage. We combine nearly 30 years of reproductive medicine experience with individualized care for every patient we see.

Consultations are held in Miami, Florida, with treatment carried out at our laboratory in Brazil; learn more about that process on our fertility tourism page. To schedule a consultation, reach out through our contact form.