In our work, we often encounter patients who do not know when to have sex in order to get pregnant. To answer this question, we must clarify three questions: When to ovulate? How to predict ovulation? How long can the sperm and egg survive after ovulation? When do you ovulate? The most important thing to know is that it is the middle of the day between two menstrual periods. If the menstrual cycle is 30 days, the ovulation time is about the 15th day after the last menstrual period; if the menstrual cycle is 28 days, the ovulation time is about the 14th day after the last menstrual period. Duan Xueguang, Department of Male Medicine, Chengde Central Hospital How to predict ovulation? There are many methods to predict ovulation, commonly used are basal body temperature method, cervical mucus examination method, ovulation test paper method, the most accurate method is ultrasound monitoring of follicles. I. The basal body temperature method is simple and convenient and can be done by the patient at home. The method is to take the oral temperature (put the thermometer in the mouth for 5 minutes) every morning after waking up before doing any activity and record it. Generally, the basal body temperature is low before ovulation and rises by about 0.5°C after ovulation until it starts to drop before the next menstrual period. This type of basal body temperature is called biphasic and indicates ovulation; if the basal body temperature always remains at one level and does not rise after ovulation, this is called monophasic basal body temperature and indicates anovulation. The patient will be able to see which type of basal body temperature is present by writing down the temperature measured every day and then connecting it to a line. II. The cervical mucus test method requires a visit to the hospital and is simple. During the menstrual cycle, the amount, viscosity, tensile strength, extensibility and properties of cervical mucus change periodically under the influence of reproductive hormones, and these changes are often used clinically to determine the presence or absence of ovulation. Under normal circumstances, the cervical mucus increases day by day in the pre-ovulatory period (equivalent to the 12th day before menstruation), and the viscosity gradually becomes thinner, until the most abundant secretion before ovulation, clear and transparent like egg white, with high extensibility that can pull out 7-10 cm long filaments, and the typical coarse lamb’s tooth crystals can be seen after coating, these performances suggest that ovulation is approaching. III. The test paper method is simpler and can be done by the patient. A test strip is sold in pharmacies, which can be placed in the urine and the changes in the test strip can be observed to determine whether ovulation has occurred. Ultrasound method. Vaginal ultrasound is equivalent to visual observation and can directly determine the growth of follicles and the time of ovulation. Usually, the vaginal ultrasound is started on the 8th to 10th day of menstruation, and the development of the follicles can be accurately determined by checking once or twice every 1 to 2 or 2 to 3 days. Generally speaking, the follicles should be about 18 to 20 mm long when you are close to ovulation. Once you know the exact time of ovulation, you can increase your chances of conception by having sex at the right time. How long can the sperm and egg survive in the body after ovulation? Under normal circumstances, the egg can survive for about 36 hours after discharge from the ovary, with the strongest fertilization ability within 15 to 20 hours of discharge, and the ability to fertilize within 36 hours. Sperm can survive for 4-5 days after being ejected into the female reproductive tract, with fertilization ability being stronger for 1 to 2 days. Therefore, only when a woman has sex within 1 to 2 days before and after ovulation is it possible to conceive; otherwise, conception is not possible.