Only child
An only child is a person who has no siblings, either biological or adopted.
Throughout history, only children were relatively uncommon. From around the middle of the 20th century, birth rates and average family sizes fell sharply, for a number of reasons including increasing costs of raising children and more women having their first child later in life.
Reasons for having an only child could include: personal preference, family planning, financial and emotional or physical health issues, desire to travel, stress in the family, educational advantages, late marriage, stability, focus, time constraints, fears over pregnancy, advanced age, illegitimate birth, infertility, divorce, and death of a sibling or parent.
Only children can later become siblings through half-siblings, step-siblings, three/fourth siblings, foster siblings, adopted siblings, co-siblings, or parents-in-law.