Ultrasound is a commonly used auxiliary method for diagnosing premature rupture of membranes. After the membranes rupture prematurely, amniotic fluid intermittently leaks out from the uterine cavity, but the production of amniotic fluid often cannot keep up with the leakage. Therefore, the majority of pregnant women with premature rupture of membranes will experience a reduction in amniotic fluid volume. Ultrasound will reveal that there is still a small amount of amniotic fluid in the amniotic cavity, but the volume is much lower than the normal range; the maximum depth of the amniotic fluid should be less than three centimeters, and the amniotic fluid index should be less than eight centimeters, which is a common occurrence. However, sometimes, if not much amniotic fluid leaks out after premature rupture of membranes, the ultrasound may sometimes show that the amniotic fluid volume is normal. In such cases, it is necessary to combine this finding with the patient's clinical symptoms to jointly determine the presence of premature rupture of membranes.