A bloody show is the passage of a small amount of blood-tinged mucus from the vagina, which occurs as the cervix begins to dilate and efface during labor. This indicates that the cervical changes necessary for labor progression are underway, signaling that delivery is approaching. It reflects disruption of cervical capillaries due to the thinning and opening of the cervix. Unlike headache, leg cramps, or nasal congestion, which can be unrelated or nonspecific, the bloody show is a direct and reliable clinical sign of imminent labor.

Reference: Williams Obstetrics, Cunningham et al., 26th Edition.