I think you are moving the bars around in search of some sort of concrete proof of in utero personhood and now you have deemed an embryo rational when it hasn’t even had a thought or awareness which is like calling the embryo of a cheetah fast. The only proven, rational person in the formula that you are conveniently leaving out of your arguments is the mother. Every molecule of the potential person belongs to her during this phase. Her heart, blood, digestive and immune systems are loaned. She is the God of motherhood, the bringer of life. If she throws herself over a cliff, it’s over.

You are confused about the chain of causality and indeed a single sperm has potential to become a human. All it needs to do is meet the right egg inside a mother who is ready and able and willing. All those other potential phases outside the womb require more than mere fertilization which is a trivial act. They 100% require gestation and birth. This is why we have birth certificates, a legal document proving personhood.