During the whole three menstrual cycle , record the length of each menstrual cycle , determine ovulation time by using basal body temperature and oviposit test paper , collect their morning urine samples at specified times ( the 4th , 7th , 10th , 13rd , 16th , 19th and 24th day of first and the third menstrual cycle ; the 4th , 5th , 6th , 7th , 9th , 12nd , 15th , 18th , 21st and 24th day of the second menstrual cycle ) , determine the concentrations of estradiol , pregnanediol and creatinine in morning urine samples ; draw the curve of the concentration changing over time and calculate the area under the curve to the 24th day .