**Core Concept**
The initiation of labor involves a complex interplay of hormonal and mechanical changes in the uterus, primarily regulated by the hypothalamic-pituitary-adrenal (HPA) axis and the autonomic nervous system. One key hormone plays a crucial role in stimulating uterine contractions, leading to labor.

**Why the Correct Answer is Right**
Oxytocin is a peptide hormone produced by the hypothalamus and secreted by the posterior pituitary gland. It acts on the uterus through oxytocin receptors (OTR), stimulating myometrial contractions and promoting labor. Oxytocin also increases the release of prostaglandins, which further enhance uterine contractions. This hormone is often administered synthetically during labor induction or augmentation to augment contractions and facilitate delivery.

**Why Each Wrong Option is Incorrect**
**Option B:** Estrogen plays a role in preparing the uterus for labor by increasing the synthesis of contractile proteins and promoting prostaglandin production. However, it does not directly initiate labor.

**Option C:** Progesterone has a relaxatory effect on the uterus and is essential for maintaining pregnancy. Its withdrawal or decrease is believed to contribute to the initiation of labor, but it does not directly stimulate uterine contractions.

**Option D:** Cortisol, a glucocorticoid hormone, has various effects on the body, including promoting fetal lung maturation. However, it does not directly influence the initiation of labor or uterine contractions.

**Clinical Pearl / High-Yield Fact**
Oxytocin's role in labor is so critical that synthetic oxytocin is often used to induce or augment labor, highlighting its importance in facilitating delivery.

**✓ Correct Answer: A. Oxytocin**