From a scientific standpoint, a newly-fertilized egg (i.e. “at conception”) is not technically alive. It cannot maintain homeostasis, and is unable to perform basic functions of life individually. From this point on, there are hundreds of different times when we could, arbitrarily, declare that a fetus has developed sufficiently to be called “alive”, but it’s a huge scientific grey area for which there is no easy answer.
From a moral standpoint, I’d say arguing that a fertilized egg is equivalent to a human being is dubious. If nothing else, this would mean that you’d have to label every fertility specialist involved in IVF a mass-murderer, as they often use many fertilized eggs to create one viable pregnancy.
“Life begins at conception” is simply a black-and-white answer to a situation that exists in shades of grey.