An ultrasound technique to measure placental growth in ewes

Abstract

Changes in mean diameter of cotyledons were measured from day 45 to 141 of pregnancy using real time ultrasound on 40 3-6-year-old Merino ewes, averaging 46 kg liveweight at day 45, and carrying one foetus. Observations were made at about 2-weekly intervals. Half of the ewes were sacrificed at day 94 and the remainder at day 141 to correlate the observations taken during pregnancy with actual data. There was a significant quadratic relationship (P < 0.001) between mean cotyledon diameter and a square root transformation of day of pregnancy. Maximum diameter of the cotyledons, as estimated by real time ultrasound, occurred at the observation on day 80 of pregnancy. The between-ewe comparison of mean diameter of the cotyledons measured by ultrasound just prior to slaughter with actual mean diameter recorded at slaughter showed a significant correlation (P < 0.001) for both the day 94 (r = 0.69) and day 141 (r = 0.71) results. Within ewes, the correlation between diameter and weight of the cotyledon was high (0.92, 0.86 for days 94 and 141 respectively), and the slope of the linear regression was less (P < 0.001) on day 141 than day 94 (0.45 v 0.32). Eighty-three per cent of the variation between ewes in total weight of cotyledons on day 94 of pregnancy was accounted for by a regression using sex of foetus, number of cotyledons and mean diameter of cotyledons measured by ultrasound on day 80 of pregnancy. In comparison, for day 141 of pregnancy 49% of the variation between ewes in total weight of cotyledons was accounted for by a regression using mean diameter on day 66 of pregnancy and number of cotyledons. The technique has potential value in studying patterns of development of the cotyledons during pregnancy in the ewe, but further validation is necessary.