Why is fertilization a poor marker for the start of personhood?
Fertilization is the start of a new cell line with a specific pattern of DNA. It's widely known that fertilization occurs when two gamete cells, a sperm and an egg, merge their haploid DNA to become a new diploid cell called a zygote. Less widely known is that no one had ever laid eyes on a human egg until the early 19th century, and no one had ever witnessed the fusion of human egg and sperm until the late 19th century (learn more here).
To be fair and just, a marker for the start of personhood must also make sense in the context of death. Fertilization as a marker for the start of personhood is problematic because we are not "unfertilized" when we die.
Some people argue that the result of successful fertilization—a zygote—is alive and has the genetic material of a complete person, therefore it is a person. This argument, that a zygote is a person because it has the DNA of a complete person, is not reasonable. To see why, consider the body you'll leave behind after you die. After your death, cells within the body you left behind can live for days. Some cells can live for more than two-weeks after you're gone, until they run out of stored energy. They eventually run out of stored energy since you are no longer there to control that body and eat or breathe to provide those cells with nutrients and oxygen. This phenomenon, of cellular life persisting in a body without personhood, is what makes life-saving organ donation possible. A dead heart is not going to save the life of someone who needs a heart transplant. Someone who needs a heart transplant needs a living, functional heart to replace their broken one. If you die and become an organ donor, some of the cells with your DNA will live for much longer than two weeks after your passing. After your donated organs are transplanted, cells with your complete DNA, that used to work to support your life, will now live, grow, work, and reproduce inside a different body to support someone else's life long after you are dead. Because cells with your DNA can live without you, the argument that “a living cell with complete human DNA is proof that a living person with that DNA exists” must be false.
Furthermore, people do not wait to declare a person dead until the very last cell with that person's specific pattern of DNA dies (that would take a loooong time, especially if they were an organ donor and had cells living on within someone else's body. That lengthy wait also means dealing with a very smelly, leaky corpse). If we are not waiting until the very last cell with a specific pattern of DNA dies to declare a person dead, then why would we do the equivalent at the start of a person's life—declaring a new person alive when the very first cell with a new specific pattern of DNA is formed?
The American College of Obstetricians and Gynecologists notes that between one-third and one-half of all embryos naturally fail to implant in the uterus and thus never develop into new human people. Of embryos that do successfully implant in a uterine wall, one-third naturally fail to develop to term.
Based on these statistics, if someone were to believe that a person's life begins at fertilization, then they'd also have to believe that two-thirds of all people on earth naturally die before they are born. And they'd have to believe that the average life expectancy of an American is not 78 years but 26 years.
Fertilization is not an appropriate marker for the start of personhood because it creates a double standard where the very first cells with a new complete set of DNA are given a right to life but the very last cells with that same complete set of DNA are not. Moreover, the consequences of that double standard are negatively profound, launching us into a dark alternate reality where the natural fate of two-thirds of all people on earth is to die before ever experiencing a breath of fresh air.