The hypothesis tested was that fetal postural development causes anterior presentation of the sheep fetus during a period of exclusive increase in anterior presentation. Exposing fetuses to a ventro-sacral position during a period of equal incidence of anterior and posterior presentations [50th-90th gestation day (GD)] should not cause changes in fetal presentation. During a period of an exclusive increase in anterior presentation (90th-140th GD) a ventro-sacral position should cause a 180° turn of the fetus. Singleton gestations with fetus in anterior presentation were included. Ultrasound examinations consisted of three consecutive two minute scans to determine fetal presentation, and were conducted with the ewe in the following three positions: the upright quadrupedal position, sitting position in which the ewe’s trunk is vertical to the ground; and again the upright quadrupedal position. The Mann-Whitney nonparametric U-test was used. On the 83rd GD (14 fetuses), there were no changes in fetal presentation, whereas on the 86th GD (10 fetuses) (p = 0.035 p < 0.05) and on the 116th GD (12 fetuses) (p = 0.000 p < 0.05) fetuses frequently assumed a transverse presentation at the second examination position. Further experiments would require the fetuses to be artificially maintained in a ventro-sacral position for a prolonged period.

Key words: fetus, presentation, sheep, postural development, gestation, ultrasound