The etymology of abortion (n.) "the expulsion of the fetus before it is viable," originally of deliberate as well as unintended miscarriages; from Latin abortionem  (nominative abortio) "miscarriage; abortion, procuring of an untimely birth," noun of action from past-participle stem of aboriri "to miscarry, be aborted, fail, disappear, pass away," a compound word used in Latin for deaths, miscarriages. The test for an unfaithful wife required her to drink a potion that could cause an abortion just to prove if she was guilty. Was it murder or the law? It was the law given by God. 

Numbers 5 :11 Then the Lord said to Moses,.... (to take this action).....Numbers 5:18 After the priest has had the woman stand before the Lord, he shall loosen her hair and place in her hands the reminder-offering, the grain offering for jealousy, while he himself holds the bitter water that brings a curse. In this case a makeshift abortion was a command and not a murder but a possible killing that has no prohibiting law. The commandment Exodus 20:13 (KJV). 13 Thou shalt not kill is wrong and biblical scholars hasten to point out that to kill is not the correct translation, the word is murder...

An abortion was not considered a murder but a killing which had no laws otherwise war would be prohibited. If we go by the letter of the law an abortion was not a sin but a legal Mosiac tool to find a woman's guilt or innocence. Some will say God, is the giver of life therefore you cannot take one, but in Numbers 5 God gave Moses instructions on what to do involving a woman's test for unfaithfulness that involves a potion that could cause an abortion just by being toxic to some innocent women's stomachs.

Conclusion: Abortion was never stated as a sin or murder but authorized by God to test a woman's guilt or innocence. Culture has assigned the definition murder to it not the Bible because the Bible used it as a tool. The only time an abortion becomes a murder is when someone intentional or unintentionally takes the life of a woman's baby in the womb. Exodus 21:22-25 - If men strive, and hurt a woman with child, so that her fruit depart [from her], and yet no mischief follow: he shall be surely punished, according as the woman's husband will lay upon him; and he shall pay as the judges determine. Hitler aborted babies in his concentration camps which is classified as premeditated murder because the unborn weren't his.