The difference between abortion and miscarriage is exactly the same as the difference between murder and accidental death. One is a deliberate, intentional killing of a person. The other is an unintended death resulting from chance circumstances. One involves people taking it upon themselves to end a life, the other is a death resulting from inadvertent situations.
Why does God allow miscarriage if He hates abortion?
An abortion is a deliberate action, taken with the specific intent of ending a human life. There is no moral comparison to a miscarriage, any more than there is one between arson and an accidental house fire. It would be ridiculous to excuse an arsonist on the grounds that some houses burn down by accident. Abortion is not made any more moral by referring to miscarriage, any more than murder can be justified by referring to accidental deaths.
Miscarriages occur when something triggers a premature birth. Often, this happens early enough in the pregnancy that the unborn child does not survive. The cause might be an illness, an injury, or some genetic issue with the child itself. It may even be due to the negligence or carelessness of the mother or some other person. Miscarriages are unintentional, and this makes all the difference.
God designed the universe to operate under a set of rules and natural laws. He only interferes with the "normal" operation of nature under extraordinary circumstances. The question of why God does not prevent miscarriages is not much different than the question of why God does not prevent any other evil—or all evil, for that matter. This draws in the question of free will and why we are allowed to make certain choices.
Part of the confusion we often have is the assumption that God is doing nothing to restrain evil. Scripture, in fact, suggests the opposite (2 Thessalonians 2:3–7). It's reasonable to believe that God has, in fact, limited the extent to which we can suffer in this world (1 Corinthians 10:13). The fact that miscarriages occur doesn't mean God has left the world unchecked. It does, however, mean that we are living in a world of cause and effect, where our choices really do matter.
God can allow for time and chance to influence what happens to us on earth (Ecclesiastes 9:11; Luke 10:31), while still condemning the purposeful murder of an innocent (Proverbs 6:16–17).
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Truth about Worldview and Apologetics