Birth Definitions

Diamniotic Twins Definition


Diamniotic twins are two fetuses that develop independently in their own amniotic sacs. They are also known as dichorionic twins. This is the most common type of multiple pregnancy, occurring in about 70% of cases.

Diamniotic twins are usually diagnosed during an ultrasound scan at around 12 weeks of pregnancy. The presence of two separate amniotic sacs and two yolk sacs is diagnostic of diamniotic twins. However, it may not be possible to confirm the diagnosis until later in the pregnancy as the embryos can sometimes move around and change position.

What are its risk factors?

The risk factors for diamniotic twins are the same as for any multiple pregnancy: advanced maternal age, a history of fertility treatments, and a family history of multiple births. 

What complications are associated with it?

The main complication associated with diamniotic twins is preterm labor and preeclampsia. Because there are two babies, each with its own placenta, the risk of placental abruption (when the placenta separates from the uterus) is also increased. Additionally, some other complications can occur in diamniotic twin pregnancies, such as Twin-to-twin transfusion syndrome (TTTS), a condition where blood flow between the fetuses is unequal, leading to one fetus receiving more blood and nutrients than the other; and selective intrauterine growth restriction (sIUGR), a condition where one or both babies do not grow properly in utero.


Diamniotic twins should not be confused with monoamniotic twins, which occur when both fetuses share the same amniotic sac. Monoamniotic twins are considered high-risk because they have an increased risk of cord entanglement, which can lead to serious complications or even death. Diamniotic twins are twins that develop from two separate eggs and two separate sperm, with each embryo developing in its own sac. It is the most common type of twin pregnancy, accounting for about 70% of all twin pregnancies. Diamniotic twins can either be fraternal (dizygotic) or identical (monozygotic).

Other Related Terms

Some terms closely associated with diamniotic twins are chorionicity and zygosity. Chorionicity refers to the number of chorions ( fertilized egg membranes) present in pregnancy. Zygosity refers to the genetic similarity of the twins.

This image shows how often the term ‘Diamniotic Twins’ is used in relation to other, similar birth terms:

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