Ok, I will directly answer your question.
Spontaneous abortion - Act of nature / act of GOD. Nothing we can do about it.
Pregnant women slaves - They made their choice. It is not a matter of making women slaves, it is a matter of holding them accountable for their actions.
People give up certain rights when they commit certain actions. The idea that a personals rights can only be taken away by due process is BS. A person can take away their rights by their own hand.
- Play in the middle of a highway - You forfeit your right to life by your own hand.
- Jump out of a plane without a parachute - You forfeit your right to life by your own hand.
- Get pregnant - You forfeit your rights by your own hand.
The unborn child did NOTHING to forfeit their right to life.
Birth is a subjective barrier to rights. It is no different than being white to vote, being a free slave, being a property owner to vote... etc. Birth is a subjective an moveable barrier.
Creation is not a subjective barrier. Once the egg attaches to the uterus and cells begin to divide, it is a life. There is nothing subjective about that. By the very definition of life, dividing cells consuming nutrients is alive.
As Jefferson said in the Declaration of Independence - "We hold these Truths to be self-evident, that all Men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty, and the Pursuit of Happiness."
For all people to be created equal, rights have to start at the time of creation.