Birth Definitions

Discordant Twins Definition


Discordant twins are two twins that significantly differ in size, and in weight, or sex. They may also differ in the type of twins they are, with one being fraternal (dizygotic) and the other being identical (monozygotic). Discordant twins are more common than most people think, occurring in about 20-30% of all twin pregnancies.

There are several reasons why discordant twins may occur. One reason is simply because the twins are not the same gestational age. If one twin is conceived a few days or weeks before the other, they will naturally be smaller and weigh less at birth. This is the most common type of discordance and is nothing to be concerned about.

Another reason for discordance is if the twins have different placentas. This can happen if each twin is implanted into a separate area of the uterus or if there was a partial separation of the placenta during pregnancy. These types of discordance usually result in one twin being much larger than the other at birth.

The last type of discordance occurs when the twins are of different sexes. This happens when each twin has a different father (fraternal twins) or when one twin is a boy and the other is a girl (identical twins). Discordant twins of different sexes are usually born close to their due date and have no other health problems associated with them.

Risks Associated with the Condition

While most discordant twins are born healthy, there are some risks associated with this type of pregnancy. One risk is that of preterm labor. If one twin is much larger than the other, it can put strain on the uterus and cause labor to start early. Another risk is that of Twin-to-Twin Transfusion Syndrome (TTTS). This occurs when blood vessels in the shared placenta become entangled and one twin starts receiving too much blood while the other doesn’t get enough. TTTS can be fatal to both twins if left untreated. Other risks include intrauterine growth restriction, placental abruption, and preeclampsia. 


Disambiguation details include:

  • Dizygotic vs monozygotic twins: Dizygotic (two-egg) twins occur when two separate eggs are fertilized by two separate sperm cells. Monozygotic (one-egg) twins occur when a single egg is fertilized by a single sperm cell and then splits into two embryos. Discordant twins can be either dizygotic or monozygotic. 
  • Chorionicity: Chorionicity refers to the number of chorions (outer membranes) surrounding the developing fetuses. If there is only one chorion, the fetuses are monochorionic; if there are two chorions, the fetuses are dichorionic; if there are three chorions, the fetuses are trichorionic; etc. For example, monochorionic diamniotic (MCDA) discordant twins have their own amniotic sacs but share a chorion and placenta, while dichorionic diamniotic (DCDA) discordant twins have their own chorions, amniotic sacs, and placentas. 
  • Zygosity: Zygosity refers to whether the twins are identical (monozygotic) or fraternal (dizygotic). Discordant twins can be either monozygotic or dizygotic, but they are always opposite in sex because they developed from different eggs (dizygotic) or from different regions of a split egg (monozygotic).

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

Other Related Terms

Terms closely related to the term Discordant Twins include the following:

Monochorionic twins

Dichorionic twins

Monoamniotic twins

Diamniotic twins 

Monochorionic monoamniotic twins

Diamniotic monozygotic twins

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