Premature rupture of membranes cannot be seen by ultrasound because it is not possible to examine the rupture of membranes. The diagnosis of premature rupture of membranes is mainly based on medical history and physical examination: medical history means that the pregnant woman has sudden or continuous vaginal flow; physical examination means that we can see a clear pool of amniotic fluid in the posterior fornix when we do vaginal speculum examination, and then when we use PH test paper to test the pool of amniotic fluid, the PH test paper may turn blue. In other words, the vaginal environment of normal pregnant women is acidic, while the amniotic fluid is alkaline, once the water breaks, the amniotic pool in the posterior fornix is alkaline fluid, so the PH test paper will turn blue or the PH test will detect the change of vaginal PH value by vaginal discharge test. Ultrasound is not always good to diagnose premature rupture of membranes.