Can you be “a little bit pregnant”?
There you will find the answer to the question “when does life begin”.
If anyone will engage in the issue logically, they will be soundly defeated, so they avoid such a logical discussion where possible.
However, if you can get them to engage, ask them when life begins. If they state anything other than at conception, then you can ask how that moment is different from the moment before it.
If they refuse to state when life begins, they’re saying they don’t know, but are erring on the side of killing a human life.
Anyone that killed a hunting buddy because he shot at motion in a bush and didn’t know whether it was his buddy or not would be prosecuted for negligent homicide.