I'd say personhood begins when God determines it begins. I dont think we have absolute proof when God considers a person a person.
Total agreement!
No fertilised ova 'is' automatically a person or guaranteed to 'become' a person.
It is up to God whether an ova has a nuclei, and if it doesn't, fertilisation of such an ova results in a hydatid-form mole, which is a pre-malignant tumor which must be removed before it becomes malignant. According to those who claim 'personhood begins at conception', these tumors are 'persons' and removal of such tumors would equate to ending personhood!
God always performs a preparation process eg God made habitats and food before God made animals and people. God does the same with fertilisation.
Fertilisation of a healthy ova (containing a nucleus) initiates preparation for a possible embryo. Such a fertilised ova initially divides to become a pre-embryonic diploid zygote, which divides to form a blastocyte which may implant in the uterine wall. After implanting, the blastocyte may proceed to divide to form the germ cells that may give rise to an embryo and embryonic sac (amnion, placenta, umbilical cord etc). The embryonic phase begins when germ cells differentiate to begin producing specialised cells.
Fertilisation results in many things, but not all result in an embryo.
In anembryonic gestation (aka blighted ovum) the embryonic sac is formed, but not the embryo.
Parthenogenesis results in an embryo without fertilisation happening, so fertilisation cannot be the only grounds for 'personhood'. Jesus was 'a person' (scientists often use parthenogenesis embryos for stem cell research because such embryos are overlooked by those who claim 'fertilisation = personhood' despite some 'persons' existing who were not the result of fertilisation and many fertilised ova are never destined by God to ever be 'persons').
We don't know which ova, fertilised or not, will become 'persons' or cysts or empty embryonic sacs. I believe 'human embryos' are persons, whether from fertilisation or parthogenesis ... I believe that Jesus was a person.
I think that claiming that every fertilised ova is 'a person' is an errant assumption, considering there are so many other possibilities and so few fertilised ova naturally progress to birth, since the vast majority are defective and are usually naturally reabsorbed or spontaneaously aborted by the body, unless defective ones continue to grow eg cysts which must be surgically removed.
I would not call a cyst a person and I would not call a parthenogenesis human embryo a non-person.