How Do Twins Form? Understanding Twin Pregnancy

When people meet twins, one of the first things they notice is how similar, or different, they look. Some twins are nearly impossible to tell apart, while others look no more alike than typical siblings.

The reason comes down to how twins form early in pregnancy. Understanding this process can also help families make sense of why some twin pregnancies are monitored more closely and why extra medical support is sometimes needed.

Let’s look at how twins form and the different ways twins can develop during pregnancy.

The Two Main Types of Twins

Most twins fall into one of two categories:

• Fraternal twins
• Identical twins

Although they share the same birthday, the way they begin development is different.

Fraternal Twins: Two Eggs, Two Babies

Fraternal twins are the most common type of twins.

Normally, one egg is released during ovulation each month. Sometimes, however, two eggs are released at the same time, and each egg is fertilized by a different sperm.

This results in:

• Two separate fertilized eggs
• Two embryos developing at the same time
• Each baby having their own placenta and amniotic sac

Because they come from different eggs and sperm, fraternal twins are genetically similar in the same way typical siblings are. They may look alike or quite different.

Fraternal twins can be:

• Two girls
• Two boys
• Or one of each

Identical Twins: One Egg That Splits

Identical twins develop differently.

In this case, one egg is fertilized by one sperm, creating a single embryo. Early in development, that fertilized egg splits into two embryos, which continue to grow separately.

Because both babies come from the same fertilized egg:

• They share the same genetic material
• They are always the same sex
• They often look very similar
• They usually share the same blood type and many physical traits

Depending on when the embryo splits, identical twins may share pregnancy structures, which leads to additional medical terms you may hear during prenatal care.

Why Doctors Use Terms Like Di-Di, Mono-Di, and Mono-Mono

During ultrasound appointments, families expecting twins may hear terms such as Di-Di, Mono-Di, or Mono-Mono. These describe how twins share, or do not share, the placenta and amniotic sac.

These details matter because they influence how closely the pregnancy needs to be monitored.

Di-Di Twins (Dichorionic, Diamniotic)

In a Di-Di pregnancy:

• Each baby has their own placenta
• Each baby has their own amniotic sac

This is the most common type of twin pregnancy and generally carries the lowest risk.

All fraternal twins are Di-Di. Some identical twins are also Di-Di if the embryo splits very early.

Mono-Di Twins (Monochorionic, Diamniotic)

In a Mono-Di pregnancy:

• Twins share one placenta
• Each baby has their own amniotic sac

This occurs only in identical twins when the embryo splits slightly later.

Because the twins share one placenta, doctors usually monitor growth and blood flow more closely. Sometimes complications such as Twin-to-Twin Transfusion Syndrome (TTTS) can occur, which is why regular monitoring is important.

This is actually the most common type of identical twin pregnancy.

Mono-Mono Twins (Monochorionic, Monoamniotic)

Mono-Mono twins, often shortened to mono-mono twins, share:

• One placenta
• One amniotic sac

Both babies develop in the same space without a separating membrane. This situation is rare and requires very close medical monitoring because the babies’ umbilical cords can become tangled.

Why Twin Pregnancies Sometimes Need Extra Care

Twin pregnancies place greater demands on both the mother and developing babies. Twins are more likely to:

• Be born early
• Have lower birth weights
• Require NICU support
• Need additional pregnancy monitoring

Hearing new medical terms or needing extra appointments can feel overwhelming, but these steps are meant to support healthy outcomes for both babies and parents.

A Gentle Reminder for Parents and Caregivers

Whether twins look identical or completely different, each baby grows and develops as an individual. Twins share a womb and birthday, but each child still has their own personality, needs, and pace of development.

Raising twins can be both joyful and exhausting, especially in the early months. Support, rest, and flexibility make a real difference during this time.

References

American College of Obstetricians and Gynecologists (ACOG). Multiple Pregnancy: Twins, Triplets, and More.

National Institute of Child Health and Human Development (NICHD). What are the different types of twins?

Mayo Clinic. Twin pregnancy: What twins or multiples mean for mom.

March of Dimes. Pregnancy with Twins.

Cunningham FG et al. Williams Obstetrics. McGraw-Hill Education.