The production of PGF2a by the endometrium toward the end of the estrous cycle causes the CL to regress. In pregnancy, CL lifespan is extended due to the release of conceptus factors that prevent luteolysis. In short, establishment of pregnancy depends on the ability of the conceptus to elicit a signal and of the mother to respond appropriately. This process remains poorly understood and differs across species. In a few species, biologically relevant active factors released by the conceptus have been identified, while in others they remain unknown (Table 1).

Because multiple signals undoubtedly pass between the conceptus and the mother, maternal responses are dynamic and complex. Those MRP signals that ensure

Table 1 Summary of MRP signals and the days of recognition in domestic species


Days of recognition

Main signal(s)


14 17



12 15



7 8a



11 14

Estrogen and prolactin


12 14


extension of CL lifespan, although crucial to continu- 8. ance of the pregnancy, are only the beginning of a sequence of endocrine, paracrine, and autocrine signals that direct changes in maternal physiology and ensure a successful pregnancy. 9.

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