The leucorrhoea appears stretched, usually 2 weeks after the onset of menstruation. After menstruation, estrogen levels are low, leukorrhea is minimal, and the vulva is nearly dry. After 3 days of menstruation, the estrogen level gradually increases, stimulating the vaginal secretion of leucorrhea, which is initially sticky and thick. The appearance of leucorrhoea is a sign that ovulation is about to occur. After ovulation, the corpus luteum is formed and enters the luteal phase. If you are pregnant, the corpus luteum does not atrophy and supports the pregnancy process; if you are not pregnant, the corpus luteum atrophies and menstruation follows. If you have irregular menstruation and cannot observe leucorrhoea, it is not easy to determine the time of menstruation. You can take blood test for progesterone level to assess the function of the corpus luteum and thus infer the time of menstruation.