IMPLANTATION begins around Day 40 and is not complete until around Day 140 when formation of the micro-cotyledons is complete. Attachment of the placenta is gradual in the mare and is unable to be precisely temporally defined
ENDOMETRIAL CUPS: Distinctive, irregular-shaped areas which develop to become grossly visible arranged in a ring at the base of the pregnant horn. Begin to produce equine chorionic gonadotrophin (eCG; old term PMSG) and appears in the circulation around Day 35. Values rise rapidly to peak around 65 Days. There is then a variable rate of decline, but may persist as long as 150 days. This effectively means the mare cannot be bred again that year once she is positive for eCG. eCG is responsible for ovulation and/or luteinisation of secondary ovulations.
Progesterone Sources: Until Day 200: primary and secondary corpora lutea
From Day 60 : Feto-Placental Unit produces progestins and gradually assumes the main role
Equine Uterine changes: becomes tonic (17 -21 Days) and swelling develops about 21 - 24 Days; by 30 Days the embryonic vesicle is 3 to 4 cm in diameter and by 40 Days is the size of a tennis ball; between Days 60 and 100, the uterus is low within the abdomen and the foetus can not usually be palpated; from 4 to 5 months onwards the foetus can usually be palpated
Equine Cervix changes: tight, firm and tonic (as during dioestrus)
Ovarian changes: 18 to 40 Days - many follicles up to 3 Cm, occasional ovulations; 40 to 120 Days - extensive ovarian activity with ovulations; luteinisation and development of secondary corpora lutea; 120 days to term: small and inactive, difficult to palpate after 5 months
Duration of Equine pregnancy is 330 to 345 Days, but enormous variation is possible and anywhere from 315 to 360 Days is frequently reported