Gynecologic ultrasound can predict the onset of menstruation based on ovulation and the thickness of the lining. However, because the variation of the thickness of the lining varies from person to person and because it is not possible to perform ultrasound every day to observe and record the date of ovulation, it is usually impossible to project an absolutely accurate time for the start of the next menstruation. Generally, menstruation begins 14 days after ovulation, but the time of ovulation can rarely be detected in a single examination and must be observed by ultrasound over a period of time. According to big statistics, the thickness of the uterine lining before menstruation is about 12mm and the thickness of the uterine lining during ovulation is about 9mm. However, not everyone’s endometrium is exactly the same and has individual specificity. Even for the same person, there is variability in the thickness of the endometrium from one menstrual cycle to another. In summary, a single ultrasound cannot predict when a period will occur, unless ovulation and endometrial conditions are monitored continuously, which makes it possible to estimate the approximate time of menstruation.