A detailed study of more than 100 Beech martens (Martes foina) captured in the Bordeaux region of France has revealed that this species exhibits a prolonged eight month period of delayed implantation which is initiated shortly after mating in July and is terminated in late February. Implantation is followed by a period of normal gestation lasting about 30 days. Birth of the young at the end of March/beginning of April is followed by a period of lactation which may last until mid-May.
The Beech marten appears to conform to the pattern exhibited by other mustelids with delayed implantation in possessing inactive corpora lutea throughout the delay phase. The resumption of normal embryonic growth is preceded by a phase of luteal activation during which the corpora lutea increase in size and vascularity the luteal cells hypertrophy and the plasma concentration of progesterone increases significantly.