A drop in body temperature on the eighth day after ovulation can indicate the presence of luteal insufficiency, which can be caused by physiological factors such as menarche, or pathological conditions such as luteal insufficiency. 1. Physiological factors: a drop in body temperature on the eighth day after ovulation, i.e., when the basal body temperature is lower than 11 days, suggests that there may be luteal insufficiency. Luteal insufficiency can be caused by physiological conditions such as menarche, post-delivery and menopausal transition. 2. Pathological factors: luteal insufficiency is caused by insufficient secretion of follicle-stimulating hormone, luteinizing hormone, poor development of the ovary itself, and luteinization of granulosa cells. Pathological luteal insufficiency may be accompanied by difficulty in conception or a history of early miscarriage. It is recommended to consult a doctor for ultrasonography and other related tests to clarify the situation.