Ovarian Changes during the Estrous Cycle and Pregnancy in Mares

Pregnant pony mares (weight, 115 to 300 kg) were necropsied on the following days after the end of estrus (number of mares in parentheses): 10 (three), 14 (four), 18 (four), 24 (three), 30 (two), 40 (two), 50 (three), 60 (three), 80 (three), 100 (three), 160 (two) and 220 (one). The primary corpus luteum was marked with India ink on day 2 or 3 in the mares which were necropsied on days 24 to 220. In a second experiment pregnant mares were necropsied and the stage of pregnancy was estimated as follows: 150± 15 days (six mares), 180 ±15 days (four mares), 210±15 days (four mares). The weight of the primary corpus luteum in pregnant mares was significantly less at day 14 than at day 10 and tended (P<10) to be greater at day 18 than at day 14. The primary corpus luteum, based on color, was maintained until day 160 or 180. Secondary corpora lutea began to appear on days 40 to 60 and the numbers increased gradually until day 160. Within mares, the primary corpus luteum and secondary corpora lutea appeared to regress at approximately the same time and only remnants of luteal structures remained by days 180 to 220. Diameter of largest follicle was greatest at day 60 and decreased thereafter as pregnancy advanced reaching smallest diameter by day 160 to 180. There was a gradual decrease in number of large follicles (>10 mm) after, day 60 and the decrease could be accounted for, at least in part, by the increasing number of secondary corpora lutea.