The size of the follicle has reached 18x16mm, which means that it is already a mature follicle and may ovulate at any time. However, if you want to grasp the time of intercourse, generally need to be after the egg is determined to be discharged, and want to accurately determine the time of ovulation, there are two ways. 1, directly through the ultrasound for the detection of ovulation, you can visually see the shape of the follicle, the size of the follicle, as well as the situation of ovulation, you can be sure to arrange for coitus within 36 hours of ovulation, the likelihood of pregnancy is very high. 2, through the ovulation test paper to detect if the follicle is mature, then you can continuously through the ovulation test paper, you can interval six hours. If a mature follicle is detected by ultrasound, it can be detected by ovulation test paper continuously, and the test can be conducted at intervals of 6 hours. Generally speaking, if the ovulation test paper appears strong positive continuously and turns into weak positive, it means ovulation, and you can arrange intercourse within 36 hours, and the possibility of pregnancy is also very high.