Twin growth discordance

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 10^th centile.

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 categorised 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 anaemia and it can happen as a result of twin-twin transfusion.

Associations

• 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
• placental insufficiency
• twin-twin transfusion syndrome
• higher incidence with a velamentous cord insertion ⁶
• higher incidence with single umbilical artery ⁶

Radiographic assessment

Antenatal 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 20mm20 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)

Prognosis

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

See also

• disparate twin growth
• amniotic fluid discordance
• automatic online twin growth discordance calculator as of 2011
• embryonic growth discordance