The Pythagorean Greeks are the first society that I find record of believing that the soul enters the body at the time of conception.
Brahman scriptures allowed abortion until the fifth month, when movement can be felt.
In Jewish tradition, the Talmud references the foetus as part of its mother, and in terms of the laws of mourning and similar statements is actually not considered a living being until its 30th day after birth. However, it gets more complex than this. Exodus 21:22 seems to give clear path to abortion until one looks at the Septaguint as an alternate translation to the Bible as we know it where "ason" is interpreted as "form" rather than "harm", which would indicate that abortion would be illegal if there were any form to the foetus, which, as will be shown later, Augustine latched on to.
Different factions of Judaism go back and forth on this, all agreeing that the life of the mother (as in the case of therapeutic abortions) transcends the life of the foetus, but some prohibiting the use of abortion on the request of the mother or in the case of a malformed foetus. There seems to be an undercurrent that the foetus is generally considered a parasite on the mother's body. References and further research available on request.
Lots of commentary in these texts is found referencing Rome's anti-abortion laws (which came in about 200 CE) as a political rather than ethical in motivation.
The Catholic church latched on to the Pythagorean beliefs in the third century (references made to the writings of Tertullian and later St. Gregory of Nyssa). Augustine reverted back to the earlier position that only the killing of a formed foetus (and this is argued at either 40 or 80 days) was murder. The Justinian code, in the sixth century, said 40 days, and this was reaffirmed several times, although the 40 and 80 days thing got kinda confused.
Sixtus V did away with this in 1588 in the bull "Effraenatum", and was almost immediately contradicted in 1591 by Gregory XIV.
It was not reversed until 1869, 39 years after abortion became a crime in the United States, by Pius IX, around the same time that similar decrees were made in most European countries.
Several references note that in some of these statements injury to the foetus was not even allowed to save the life of the mother, although I cannot find specifics in my limited library nor do I have the Canon Law, instituted in 1918, to find the current standing on saving the life of the mother. However, in a contradictory position, Catholicism in general has treated the foetus as part of the mother in terms of punishment of crimes.
I don't have as many references to the protestant Christians or other religious groups as I'd like, I'd appreciate it if people would fill me in, and I'd also like more references on other stands and such since the mid 1800s.