There is no consensus on the definition of life, let alone whether it begins at conception.
At fertilization, a single-celled organism known as a zygote forms. However, this entity does not begin to resemble the human form until about week nine of gestation. Furthermore, a fetus typically cannot survive outside of the mother's womb until the third trimester of the pregnancy.
While it is uncertain when exactly human characteristics such as self-consciousness and the ability to feel pain emerge, the brain and nervous system of the fetus must be developed before these capacities are possible.
Scientists, bioethicists, philosophers and legislators disagree on which of these criteria constitute "life" or "personhood," as reflected in different definitions, theories and laws.