On doppler the most omnious sign indicating fetal compromise is:
Doppler ultrasound is used to assess the uterine, placental and fetal arterial and venous system.
It is especially relevant in IUGR.

Ratios measured by Doppler:

S/D – Systolic - diastolic ratio or S/D ratio (most commonly)
S-D/S – Resistance index
S-D/Mean – Pulsatility index

Umbilical artery vessels:

Blood flow across the umbilical arteries provides a comprehensive overview of the - fetal blood supply and helps in detecting fetal circulatory compromise.
The systolic component of blood flow reflects the cardiac pump and the diastolic component the distal vascular bed.
In this case the distal vascular bed is the placental villous tree and hence umbilical artery Doppler is actually a placental and not a fetal Doppler.
The umbilical artery is characterised by forward low resistance flow.
As gestation increases, there is gradual fall in all the resistance indices so the amount of forward diastolic flow increases (Remember : Diastolic flow is inversely related to peripheral resistance). systolic/diastolic rates decreases as period of gestation increases.
↑ pulsatality index in umblical art
↑S/D blood flow ratio
↑Cerebral arter flow
Absent diastolic flow