Diagnostic thresholds for distinguishing complicated disorders that occur in monochorionic multifetal pregnancies

Disorder	Diagnostic criteria
Twin-twin transfusion syndrome (TTTS)	Amniotic fluid volume discordance: Donor MVP <2 cm Recipient MVP >8 cm
Twin anemia polycythemia sequence (TAPS)	MCA-PSV discordance: Donor <1.0 MoM with recipient >1.5 MoM (however, the diagnostic criteria for TAPS are evolving toward MCA-PSVs consistent with anemia in one twin and either a decrease or significant discordance in MCA-PSV for the other twin rather than one particular cut-off) Placental findings on ultrasound: Donor (anemic) region: Hyperechoic and thickened Recipient (plethoric) region: Normal appearance
Selective fetal growth restriction (sFGR)	Estimated fetal weight discordance >25% with small twin <10^th percentile Type based on UA Doppler velocimetry: Type I: Normal flow Type II: Absent or reversed end-diastolic flow Type III: Intermittent absent or reversed end-diastolic flow

For additional information, refer to UpToDate content on TTTS, TAPS, and sFGR.

MCA: middle cerebral artery; MoM: multiples of median; MVP: maximum vertical pocket; PSV: peak systolic velocity; UA: umbilical artery.

Modified from:

Khalil A, Rodgers M, Baschat A, et al. ISUOG Practice Guidelines: role of ultrasound in twin pregnancy. Ultrasound Obstet Gynecol 2016; 47:247.
Society for Maternal-Fetal Medicine, Simpson LL. Twin-twin transfusion syndrome. Am J Obstet Gynecol 2013; 208:3.
Sueters M, Middeldorp JM, Lopriore E, et al. Timely diagnosis of twin-to-twin transfusion syndrome in monochorionic twin pregnancies by biweekly sonography combined with patient instruction to report onset of symptoms. Ultrasound Obstet Gynecol 2006; 28:659.

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