This is a special pleading; It's therfore invalid.
The only things nec. to move from adult to oldster, from teenager > adult, from child>teen, todler>child, baby>todler, fetus to baby, embryo to fetus, or from zygote to embryo is time, nutrition, shelter, hydration & gas exchange CO2 out, O2 in).
Each of those is the name of a stage of development for some unifying "thing" that is doing the progression through those stages. That "thing" is a human being.
This is not true of one who is brain dead. No amount of time is going to lead to their recovery — they are gone.
Also, this is not true of the component parts, prior to fertilization.