(A) Type 1 sFGR. Persistently forward umbilical artery end-diastolic flow without variation in the waveform and normal or elevated resistance. (B) Type 2 sFGR. Fixed absent or fixed reversed umbilical artery end-diastolic flow without variation of the waveform in the smaller twin. (C) Type 3 sFGR. Variable umbilical artery flow pattern that cycles between forward, absent, and reversed flow over a short interval.
