The changes in the plasma levels of progesterone, of unconjugated oestrone and oestradiol-17 β, and of oestrone sulphate in the sow at the time of implantation, during gestation and at parturition are described. The progesterone concentration remained fairly constant (10 to 12 ng/ml) throughout pregnancy until 20 to 15 days before parturition. The progesterone concentration dropped over the last 15 days of gestation. Measurable quantities of oestrone sulphate were present in the maternal plasma at Day 16 but not at Day 9 of gestation and rose to a peak >3 ng/ml (assessed as free oestrone) between Day 23 and Day 30. The concentration fell to a low level of 35 pg/ml on Day 46 before rising to a second peak of 3 ng/ml just before parturition. Measurable quantities of unconjugated oestrone (>10 pg/ml) and oestradiol-17 β (>15 pg/ml) first appeared in the plasma at approximately Day 70 to Day 80 and their rise to a peak at the time of parturition followed very closely that of oestrone sulphate. Unconjugated oestrone rose to a peak at a mean concentration of 2·5 ng/ml and oestradiol-17 β at 400 pg/ml. No oestradiol-17α was detected. The concentration of these steroids returned to basal levels by 72 hr after parturition.
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