True labor begins when which of the following occurs?

True labor is characterized by specific physiological changes that prepare the body for childbirth. The presence of a bloody show is significant because it indicates that the cervix is beginning to efface and dilate in preparation for labor. This bloody show occurs when the mucous plug, which seals the cervical canal during pregnancy, is expelled, suggesting that the body is transitioning into the active stages of labor.

In contrast, irregular contractions, pain without cervical change, and full dilation point to different stages or conditions in the labor process. Irregular contractions might occur during Braxton Hicks or false labor, where there is no significant cervical change. Feeling pain but having no change in the cervix indicates that labor has not fully commenced. Additionally, a fully dilated cervix is more indicative of the end stage of labor rather than the onset.

Thus, the correct answer highlights a key sign that true labor has begun, namely the presence of a bloody show, reflecting cervical changes critical for the transition to active labor.