Table 1.

A chronology of studies providing evidence on timing of maternal recognition of pregnancy in the horse.

Reference	Time examined (days post-ovulation)	Relevant outcome	Experimental approach	Results relevant to MRP	Implications for MRP timing
Leith & Ginther (1984)	Days 9 to 17	Mobility patterns of the embryonic vesicle	Ultrasonography of pregnant mares	On day 9, conceptus mobility was minimal. Mobility increased on day 10 and reached an apparent plateau from day 11 to 14.	Suggests MRP signalling active during days 10–14
Sharp et al. (1984)	Days 4 to 20	Endometrial and uterine PGF content; response of endometrium to presence of conceptus	PGF measured in uterine luminal flushings and endometrium; in vitro incubations of endometrium alone vs endometrium in the presence of conceptus membranes.	Uterine luminal PGF and endometrial PGF/in vitro production peaked on/around day 14 in cycling mares. Endometrial PGF content remained lower in pregnant mares until day 20, but in vitro production capability of endometria from pregnant mares continued to increase post day 14. Co-incubation of endometrial tissue and conceptus membranes in vitro reduced PGF production.	Embryos exert MRP by blocking PGF production; the process is engaged before day 14; day 14 endometrium is responsive to MRP.
Goff et al. (1987)	Days 9 to 14	Oxytocin responsiveness of endometrium	Pregnant and non-pregnant mares were given oxytocin intravenously and plasma PGF metabolites measured.	In non-pregnant mares, the increase in plasma PGFM response to oxytocin was greater at day 13 than day 11; no significant increase in response to oxytocin days 9–14 in pregnant animals.	Non-pregnant endometrium becomes responsive to oxytocin between day 11 and 13; MRP expected to be initiated before this time.
Sissener et al. (1996)	Conceptus days 9, 12, 13, and 16; uterine biopsies on day 14.	Age of the conceptus inducing maximal suppression of PGF2α	Embryos recovered and co-incubated with endometrium, PGF2α measured in all samples.	Day 12 conceptuses significantly suppressed endometrial PGF2α secretion compared with that of endometrial tissue incubated alone.	Embryo emits MRP signal at day 12 (other timepoints cannot be excluded).
Starbuck et al. (1998)	Days 10, 14, 18	Oxytocin responsiveness of endometrium; role of oxytocin receptors	Pregnant and non-pregnant mares injected with oxytocin; circulating concentrations of PGFM measured; concentrations of oxytocin receptor measured in endometrial biopsy samples.	PGFM concentrations were increased after oxytocin administration on days 10/14/18 in cyclic mares, but only on days 10/18 in pregnancy. Suppression of oxytocin receptors in the endometrium occurred between day 10 and 16.	Oxytocin responsiveness/receptor expression is blocked in pregnant mares after day 10 and before day 16, suggesting possible window of MRP.
Stout et al. (1999)	Days 8 to 20	Oxytocin responsiveness of endometrium	Oxytocin vs saline administered to mares via continuous infusion, monitored for luteolysis.	Continuous administration of oxytocin on days 8–20 abolished luteolysis in most mares. When oxytocin treatment began on day 10, instead of day 8, luteolysis occurred in two of five mares.	Oxytocin responsiveness begins on or about day 10.
Ababneh et al. (2000)	Day 13 embryos	Ability of embryos to suppress endometrial PGF2α	Embryos were collected from pregnant mares 13 +/—− 0.5 days after ovulation and cultured for 24 h; conceptus media assayed for antiluteolytic activity by measuring endometrial PGF2α synthesis in vitro.	Antiluteolytic activity detected in embryo-conditioned culture medium at 12, 18 and 24 h.	Day 13 conceptuses are capable of emitting MRP signal.
Stout & Allen (2002)	Days 14–18 (cycling mares); days 12–32 (pregnant mares).	Effect of pregnancy on uterine luminal prostaglandin levels	Influence of pregnancy on concentrations of prostaglandins in the uterine lumen examined	PGF2α concentrations reached high values in uterine flushings recovered from cyclic mares during days 14–16 after ovulation, but were negligible in flushings from pregnant mares at this time.	Conceptus attains MRP during days 12–16.
Boerboom et al. (2004)	Days 10, 13, and 15	Response of endometrium to presence of conceptus.	Prostaglandin synthesis enzymes examined in endometrial biopsies obtained from cycling mares on days 10, 13 and 15; pregnant mares on day 15.	COX-2 expression higher at day 15 in cycling endometrium relative to other timepoints. COX-2 expression in day 15 pregnant endometrium was similar to day 10 and day 13 cycling animals, suggesting that presence of the conceptus blocks the induction of COX-2.	MRP has been activated by day 15 via COX-2 suppression
Ealy et al. (2010)	Day 14	Ability of embryos to suppress endometrial PGHS2	Relative abundance of PGHS2 mRNA measured in endometrium derived from estrous cyclic and pregnant mares on day 14 post-ovulation, co-incubated with conceptus-conditioned medium.	Exposing endometrial explants to conceptus secretions decreased PGHS2 mRNA abundance.	Endometrium is responsive to PGF synthesis inhibition on day 14.
Wilsher et al. (2010)		Ability of mare to recognise pregnancy following asynchronous embryo transfer	Day 10 horse embryos were transferred non-surgically to recipient mares that had ovulated 9, 7, 6, 5, 4, 3, 2 or 1 day after, on the same day, or 2 or 4 days before the donor.	Luteostasis was achieved in recipient mares when day 10 embryos were transferred to recipient mares at any stage of asynchrony between −9 and +2 days with respect to the donor, i.e. there is a wide window for establishment of pregnancy following embryo transfer to asynchronous recipients.	The luteolytic cascade in the non-pregnant mare is not initiated before day 12 after ovulation.
Wilsher & Allen (2011)	Days 6, 8, 10, 12, 14	Endometrial responsiveness to PGF synthesis inhibition	Oestradiol and/or oils infused into the uterine lumen of cycling mares on days 6, 8, 10, 12 or 14 post-ovulation; other mares received intrauterine infusion of fractionated coconut oil, peanut oil, mineral oil or oestradiol in mineral oil on day 10 post-ovulation. Luteal function monitored by plasma progesterone assay.	Intrauterine administration of oestradiol in coconut oil delayed luteolysis at days 8, 10, 12, or 14 but was less effective at day 6. Coconut oil alone or peanut oil administered at day 10 induced the same high rate of luteal persistence. A possible interpretation is that fatty acid inhibition of phospholipase A2 limits the ability of endometrium to synthesise prostaglandins.	Endometrium appears responsive to PGF synthesis inhibition on days 8–14 but not on day 6.
Camozzato et al. 2019	Days 7, 10, 13	Histological changes in the endometrium of pregnant vs cycling mares	Endometrial biopsies from pregnant and non-pregnant mares assessed histologically; plasma progesterone concentrations measured daily.	In pregnant vs non-pregnant mares, differences in vasculature observed on days 7–13, differences in endometrial histology by day 7, differences in histotroph secretion by day 10. No difference in progesterone levels in pregnant vs cycling mares.	Some form of embryo-maternal dialogue is occurring from at least day 7.
Klohonatz et al. (2019)	Days 9 and 11	Ability of embryos to suppress endometrial PGF2α	Endometrial biopsies from pregnant and non-pregnant mares on days 9 and 11 co-incubated in contact with embryos of corresponding age; PGF measured in culture medium.	In day 11 samples from non-pregnant mares, presence of an embryo decreased PGF secretion compared to control samples from non-pregnant mares. On day 9, there was no change in PGF secretion in the presence of an embryo.	MRP occurring at day 11 but not yet detectable by this method at day 9.

COX-2, cyclooxygenase 2; PGF, prostaglandin F; PGFM, prostaglandin metabolite, 13,14-dihydro-15-ketoprostaglandin F2 alpha ; PGHS2, prostaglandin G/H synthase 2.