A heartbeat is the measurable standard by which a condemned prisoner is rated as either alive or dead. I don’t see why the same standard can’t be used with regards to a baby.
What does a pregnancy test prove? That there is another life in the womb. Find a non-invasive way to detect the first signs of pregnancy, because that means there is a new set of human DNA which also proves guilt or innocence in many life or death sentences for crimes.