A 35-year-old multigravida with chronic hypertension is found to have IUGR at 30 weeks. Umbilical artery Doppler shows elevated pulsatility index (PI 1.8, normal <1.3) with preserved end-diastolic velocity. Middle cerebral artery Doppler is normal. Amniotic fluid index is 8 cm. Which single finding best distinguishes early-stage pathological IUGR from late-stage IUGR with severe placental insufficiency?