Contents To determine the effects of poor maternal nutrition and litter size on foetal growth during mid-gestation, pregnant ewes (n=82) were fed 100%, 60% or 140% of NRC TDN beginning at day 30.2 +/- 0.2 of gestation. Transabdominal ultrasound was performed weekly between day 46.0 +/- 0.4 and 86.0 +/- 0.7 to monitor foetal heart width (HW), umbilical diameter (UMB), rib width (RW) and placentome outer (OD) and inner diameter (ID). Data were analysed with repeated-measures using the mixed procedure for effects of maternal diet, litter size and gestation, and equations predictive of gestational age were generated using the regression procedure. To determine the agreement of ultrasound measurement and actual size, ewes (n=20-21) were euthanized at day 45 or 90 to obtain corresponding postmortem measurements for Bland-Altman analysis. The HW, UMB and placentome OD and ID increased with gestation (p<.0001) but were unaffected by maternal diet or litter size (p.12). Ultrasound underestimated postmortem measurements of HW (14.8%), UMB (7.3%), placentome OD (4.5%) and ID (37.3%) at day 90 of gestation. Ultrasound underestimated RW at day 45 (7.7%) but overestimated RW (23.8%) at day 90, indicating inconsistent bias when reporting RW by ultrasound. Combining the HW, UMB, RW and placentome OD generated the strongest equation predictive of gestational age (R-2=.91). These findings indicate that during mid-gestation, maternal diet or litter size did not affect HW, UMB or placentome diameters and these factors can be used to estimate gestational age.