Exodus 20:13 New International Version:
You shall not murder.
LXX:
Οὐ φονεύσεις [G5407].
Is aborting a baby murder?
The author of the Didache (1st century AD) thought so,
Didache 2:
2 ου φονευσεις [G5407], ου μοιχευσεις, ου παιδοφθορησεις, ου πορνευσεις, ου κλεψεις, ου μαγευσεις, ου φαρμακευσεις, ου **φονευσεις τεκνον** εν φθορα ουδε γεννηθεν αποκτενεις.
not you will murder , not you will do adultry , not you will do child sex, not you will do illicit sex, not you will steal , not you will do magic, not you will do sorcery , not you will murder child to destruction neither newly begotten in the womb you will kill.
Lightfoot translated the Greek into the English "abortion":
{Thou shalt do no murder, thou shalt not commit adultery,} thou shalt not corrupt boys, thou shalt not commit fornication, {thou shalt not steal,} thou shalt not deal in magic, thou shalt do no sorcery, thou shalt not murder a child by abortion nor kill them when born,
Elsewhere in Numbers 5:
27 If she has made herself impure and been unfaithful to her husband, this will be the result: When she is made to drink the water that brings a curse and causes bitter suffering, it will enter her, her abdomen will swell and her womb will miscarry, and she will become a curse. 28 If, however, the woman has not made herself impure, but is clean, she will be cleared of guilt and will be able to have children.
Did God cause abortion in this case?
I think the issue is more complex than simply the two extreme positions of pro-life and pro-choice. See
When does soul life begin in the womb?.