**Core Concept**
The onset of labor is a complex process involving multiple physiological changes in the female reproductive system. The primary goal of labor is to facilitate the delivery of the fetus through the birth canal. The process begins with the initiation of uterine contractions and cervical changes, ultimately leading to the expulsion of the fetus.

**Why the Correct Answer is Right**
The formation of the bag of waters, also known as rupture of membranes, is a critical sign of the onset of labor. This occurs when the amniotic sac, which surrounds the fetus, ruptures and releases a gush of fluid. The release of amniotic fluid is a direct result of increased uterine contractions and the softening of the cervix, making it easier for the fetus to pass through the birth canal. The rupture of membranes signals the progression of labor and is often accompanied by a change in fetal heart rate patterns and an increase in uterine contractions.

**Why Each Wrong Option is Incorrect**
**Option A:** Labor pains are a symptom of labor, but they are not a specific sign of its onset. The intensity and frequency of contractions may vary throughout the labor process.
**Option B:** The show, also known as a mucous plug, is a sign of impending labor, but it is not a definitive indicator of its onset. The presence of a show can occur days or weeks before labor begins.
**Option C:** Dilatation of the internal os is a sign of labor, but it is not the initial sign. The internal os begins to dilate as labor progresses, but this change is not always noticeable until the cervix has dilated to around 3-4 cm.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the presence of a show or labor pains does not necessarily indicate that labor is imminent. The rupture of membranes is a more reliable sign of the onset of labor, and healthcare providers typically monitor for this sign when assessing women in labor.

**✓ Correct Answer: D. Formation of bag of waters**