Maternal progesterone: impacts on conceptus development and conceptus-maternal interactions

In brief: Maternal progesterone modulates the uterine environment that governs conceptus elongation, signaling, and implantation in cattle, yet supplementation strategies inconsistently improve pregnancy success. This review synthesizes current understanding of progesterone-regulated conceptus-maternal interactions and highlights the use of longitudinal biomarkers (IFNT, ISG, and PAG) to better resolve how progesterone influences early pregnancy establishment. Abstract: Progesterone is the key maternal hormone regulating uterine function during early pregnancy to support the developing conceptus and promote uterine receptivity. In turn, the conceptus acquires the capacity to signal its presence and further modulates a maternal response to allow for attachment and initiation of placentation. Supplementation of progesterone is proposed to optimize conceptus-maternal interactions and favor pregnancy establishment due to advanced temporal changes in the uterine milieu allowing for hastened conceptus elongation. Nevertheless, supplemental progesterone has not consistently increased pregnancy per service when applied in beef or dairy cattle, indicating that further research is required. Beyond increasing conceptus elongation, the extent to which supplemental progesterone impacts conceptus development and signaling to favor pregnancy establishment remains poorly understood. Evaluation of interferon tau, interferon-stimulated genes, and pregnancy-associated glycoproteins can be used to infer about the progression of the conceptus through key developmental milestones but has not been widely applied in the context of progesterone supplementation. Importantly, assessment of these conceptus- and pregnancy-specific biomarkers may be performed in vivo and can provide more adequate insight into the timing of conceptus-maternal interactions in response to supplementation of progesterone, especially when examined temporally.