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.
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Epidemiology
A twin growth discordance may occur in up to 25% of twin pregnancies. It is more common in monochorionic pregnancies.
Pathology
Depending on the parameter involvement this may be categorized as:
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weight discordance (interpair weight disparity of >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
Associations
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intrauterine growth restriction (IUGR) in the smaller twin:
depending on the definition the smaller twin must have IUGR according to some authors.
otherwise: the greater the discordance the higher the likelihood
higher incidence with a velamentous cord insertion 6
higher incidence with single umbilical artery 6
Radiographic assessment
Ultrasound
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.
Ancillary sonographic features include:
oligohydramnios in smaller twin (especially if discordance is severe)
Treatment and prognosis
Twin pairs with growth discordance in which one twin is small for gestational age, have an increased risk of neonatal death