MEAN CONCENTRATIONS OF PROGESTINS AND TOTAL ESTROGENS PRIOR TO BIRTH:

DAYS OF PARTURITION	PROGESTERONE (ng/mL)	Total Estrogens (ng/mL)
15	4.8	3300
10	8.2	2100
9	10.1	1700
8	14.1	1600
7	21.0	900
6	25.0	850
5	27.0	600
4	29.0	550
3	31.0	400
2	35.0	410
1	36.0	300

Progestagens rise and Total Estrogens fall prior to parturition in the mare. The rapid rise in Progestagens is thought to be due to corticoid secretion by the fetus. The typical relationship between Progestagens and Total Estrogens prior to foaling is shown below. Not all mares follow this pattern. Alkaloids in the fescue and other plants prevent the dramatic rise in the Progestagens. The mechanism of this action is unknown.