**Core Concept**
The family life cycle is a theoretical framework used to describe the stages that families go through from formation to dissolution. It is essential to understand these stages to identify the needs and challenges that families face at different points in their life cycle. The family life cycle stages include marriage, the birth of children, and the departure of children from the family home.

**Why the Correct Answer is Right**
The leaving home of the first child is a significant milestone in the family life cycle, marking the end of the dependent stage and the beginning of the young adult stage. This stage is characterized by a reduction in family size, increased independence of the children, and a shift in the family's focus from child-rearing to aging and retirement planning. The departure of the first child from the family home often leads to a re-evaluation of family roles, relationships, and responsibilities.

**Why Each Wrong Option is Incorrect**
**Option A:** Marriage is the initial stage of family formation, but it is not the stage of contraction of the family size.
**Option B:** The birth of the first child marks the beginning of the family's dependent stage, not the stage of contraction.
**Option C:** The birth of the last child is a significant event, but it may not necessarily lead to a reduction in family size, as the children may still be dependent on their parents.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the family life cycle is a dynamic process, and families may experience multiple transitions and challenges throughout their life cycle. Understanding these transitions can help healthcare providers tailor their services to meet the unique needs of families at different stages of their life cycle.

✓ Correct Answer: D. Leaving home of first child