Final answer:
The stage of family development that begins at the birth or adoption of the first child is known as the parenthood stage.
Step-by-step explanation:
The stage of family development that begins at the birth or adoption of the first child is known as the parenthood stage. In this stage, couples transition from being a married couple without children to becoming parents. It is a significant milestone in the family life cycle and involves new responsibilities, adjustments, and changes in the dynamics of the family.