The distinction between these terms is related to time after fertilisation. Fertilisation results in the creation of an embryo, which becomes a fetus after 45 days in cattle. Therefore, loss of pregnancy prior to 45 days is termed 'Embryonic Mortality'. Loss of pregnancy after 45 days is termed 'Abortion'.
Early embryonic mortality describes the demise of the embryo prior to 16 days post fertilisation. This is before the maternal recognition of pregnancy factor (thought to be interferon-t) is released. Thus the cow will return to oestrus after a normal oestrus interval. Around three quarters of embryonic loss is during this early period. This is usually unnoticed from a practical point of view.
Late embryonic mortality describes the demise of the embryo between 16 and 45 days post fertilisation. Although only one quarter of embryonic loss is during this period, the proportion which is noticed on-farm is much higher, as a number will be lost after a positive Pregnancy Diagnosis has been given. Abortion describes the loss of the fetus between 45 and 270 days post fertilisation.