Abortion is mainly hindsight, an outcome of consequence that was unknown until that time. Abortion could also be forethought as a known step beyond contraception. There are also some other ideas:
Bukhari Volume 4, Book 54
Narrated 'Aisha: The Prophet said, "Kill the snake with two white lines on its back, for it blinds the on-looker and causes abortion."
Narrated 'Aisha: The Prophet ordered that a short-tailed or mutilated-tailed snake (i.e. Abtar) should be killed, for it blinds the on-looker and causes abortion."
Narrated Abu Mulaika: Ibn Umar used to kill snakes, but afterwards he forbade their killing and said, "Once the Prophet pulled down a wall and saw a cast-off skin of a snake in it. He said, 'Look for the snake. 'They found it and the Prophet said, "Kill it." For this reason I used to kill snakes. Later on I met Abu Lubaba who told me the Prophet said, 'Do not kill snakes except the short-tailed or mutilated-tailed snake with two white lines on its back, for it causes abortion. So kill it.' "