Perhaps a guide would be how we have always treated other human life when there has been a doubt that it exists. We do not bury those who are doubtfully dead. We work frantically to help rescue entombed miners, a child lost in the mountains, or a person under a collapsed building. We suggest that the truly human way of thinking would be to give life the benefit of the doubt. Ever since its discovery 150 years ago, both words were used to mean the union of sperm and ovum.
Conception is the implantation of the blastocyst.
It is not synonymous with fertilization. Yellowstone Park is a person. So is General Motors. So are you. But the Supreme Court of the U.S. in 1857 ruled that black people were not persons, and in 1973 that unborn people were not persons.