Twin growth discordance

Last revised by Tom Foster on 02 Nov 2019

Twin growth discordance is a term used in obstetric imaging to describe a significant size or weight difference between the two fetuses of a twin pregnancy. To be classified as a growth discordance, some consider that the estimated fetal weight (EFW) of the smaller twin should fall under the 10th centile.

A twin growth discordance may occur in up to 25% of twin pregnancies. It is more common in monochorionic pregnancies.

Depending on the parameter involvement this may be categorized as:

  • weight discordance (interpair weight disparity of greater than 25% 2): this usually occurs when there are factors that affect the twins asymmetrically, possible risks that are associated with IUGR.
  • size discordance: this is usually associated with anemia and it can happen as a result of twin-twin transfusion.

In the early first trimester, the difference in crown-rump length (CRL) may be used as a parameter. 

During later stages size discordance is better assessed using the abdominal circumference (AC) 2,3 and the disparity of 20 mm or more are usually taken as a cut-off value for considering the pregnancy as discordant. The accuracy of using the abdominal circumference is however disputed by some authors 2.

A weight discordance is assessed by taking considering the estimated fetal weight (EFW) difference at 20-25% 2-4.

Twin pairs with growth discordance in which one twin is small for gestational age, have an increased risk of neonatal death

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