Source of oestrogen in early pregnancy in the mare

Oestrogen secretion was determined by oestrogen conjugate (EC) analysis of urine in three groups of pregnant mares: Group I (N = 6), animals ovariectomized on Day 18-19 of gestation with pregnancy maintained by daily administration of an oral progestagen, altrenogest; Group II (N = 9), untreated, pregnant mares; Group III (N = 5) intact, pregnant mares treated daily with altrenogest. The mean EC concentrations in the ovariectomized mares in Group I increased in a constant linear manner from 17 ng/mg Cr on Day 20 to 291 ng/mg Cr on Day 70, with no apparent surge in oestrogen secretion around Day 39. Mean EC concentrations on Days 33, 39 and 44 were respectively 41, 48, and 73 ng/mg Cr. In the intact mares in Groups II and III (shown in parentheses), the mean urinary EC concentrations were 201 (171) ng/mg Cr between Days 20 and 33 of gestation, increased rapidly from 172 (77) ng/mg Cr on Day 33 to a peak of 1066 (895) ng/mg Cr on Day 39, followed by a decline to 637 (719) ng/mg Cr on Day 44. After Day 44, EC concentrations continued to increase in a linear manner to 1191 (842) ng/mg Cr on Day 70. The mean EC concentrations between Days 20 and 70 in Group I were significantly (P less than 0.05) lower than in mares in Groups II and III. EC concentrations in Group III mares were significantly lower (P less than 0.05) than in Group II mares between Days 28 and 34.(ABSTRACT TRUNCATED AT 250 WORDS)