Doppler Ultrasound Indices and Fetal Biometry as Prenatal Markers of SGA or Non-SGA Developmental Trajectories in Naturally Nutrient-Restricted Sheep Pregnancies from Patagonia

In rangeland systems, natural nutrient restriction (NR) elevates risk for small-for-gestational-age (SGA) lambs, yet some NR ewes deliver Non-SGA offspring. We evaluated whether fetal biometry and Doppler indices could differentiate those fetal growth trajectories. Ninety-five single-pregnant Corriedale ewes were assigned from gestational day (GD) 70 to term to NR grazing (n=72) or supplemented Controls (n=23) groups. Fetal biparietal diameter (BPD), femur length (FL), thoracic height (TH), and umbilical cord diameter (UCD) were assessed at GD80, GD95, GD110, and GD125. Resistance (RI) and pulsatility (PI) indices were recorded for the umbilical (UA), cotyledonary (CA), and uterine (UtA) arteries at GD80, GD95, GD110, GD125, and GD140. Within NR, offspring were stratified by birth-weight (BW) quartiles to define SGA (lowest quartile, n=18) and Non-SGA (highest quartile, n=18) groups. Prenatal biometry did not differ among groups. At birth, BW differed (p < 0.05) among Control (4.95 ± 0.10 kg), Non-SGA (5.33 ± 0.06 kg), and SGA (3.79 ± 0.11 kg), and BPD and FL were smaller in SGA vs Control and Non-SGA. UA-RI at GD125 was higher in SGA vs Non-SGA (p < 0.005). UtA-RI and UtA-PI were lower in mothers of SGA lambs at GD110 and GD125 (p < 0.05). CA indices did not differ. UA-RI at GD125 was associated with BW (R2 = 0.15; p < 0.001). Associations between BW and UtA-RI and UtA-PI at GD 110 and 125 explained little variance (R2 ≤ 0.08). Distinct uteroplacental and umbilical Doppler signatures differentiated SGA from Non-SGA trajectories before measurable divergence in fetal growth. Although UA-RI at GD125 demonstrates prenatal predictive promise, its accuracy as standalone indicator remains modest.