It isn't ignored. It is simply not used as the sole determinant. As is often the case in the affairs of mankind.
Determining the moment at which an organism will change from one state to another, will not inform you as to the value that you place on that organism's existence.
In law, a foetus is not considered to be a human being, in the sense that a 'reasonable person' would define it.
What is the measurement of determining worth?
What is the definition of reasonable.
Show me some metrics.
That takes us back to the list:
At first breath
At quickening (whenever that is)
At developed brain activity (when exactly and do they mentally infirm count?)
At ensoulment (what exactly is that?)
As the mother determines
As the state determines viability (we have several of those)
When the heart starts beating
When the fetus can feel pain
Only during the first trimester
Up to the third trimester
One month after birth (extreme view of eugenicist Peter Singer)
When the fetus looks "human."
There are probably quite a few more. Which one is right?
That is why women need to be informed on what we do know as fact.
What we have as fact is scientific. The human being begins its distinct life at conception.
Whatever subjective "worth" or rationalized personhood claim is to be measured by the facts as we know them.
What do we know?
23 chromosomes from dad + 23 from mom= 46.
A distinct human being developing towards full adulthood if allowed to live.