Estimation of gestational age in Merino ewes by ultrasound measurement of fetal head size

Ultrasound measurements of fetal head length (occipito-snout diameter) and width (biparietal diameter) were made at 2-week intervals from about day 45 of pregnancy onwards in 3-6 year old Merino ewes bearing a single fetus, and at 1 week intervals from about day 36 to 86 of pregnancy in 4-5 year old Merino ewes bearing twin fetuses. The single-bearing ewes maintained liveweight (47 kg) during the study. The twin-bearing ewes (47 kg on day 30) were fed either to maintain weight or at 0.3 of this rate until about day 96 of pregnancy. A sample of ewes from both studies were slaughtered on days 92-94 of pregnancy, and comparable measurements taken. There were no significant effects of sex of the fetus, or undernutrition of the twin-bearing ewes on the head dimensions, except for effects (P<0.05) of feeding treatment on head length of the twins at about day 51, day 86 and day 92. The magnitude of these differences was small ( < 2 mm), equivalent to about 2 days' growth. Fetal head measurements were similar for single and twin fetuses up to about 80 days gestation. It was concluded that the most accurate prediction of gestational age was provided by head length of the fetus, with 95% confidence limits ranging from ¦2 days at about day 40 of pregnancy to ¦4 days at day 80 of pregnancy. A single linear prediction equation would be the most appropriate to use under field conditions. Verification of these relationships is recommended to check for differences that may exist between equipment, operators and breeds of sheep.