There is an interesting article on the historical beliefs of the Roman Catholic Church
here. You can see that opinions varied widely from age to age - even some popes were swayed by the sophism and secular culture of the Greek philosophers. Here are a few quotes:
Pope Innocent III (?-1216) wrote a letter which ruled on a case of a Carthusian monk who had arranged for his female lover to obtain an abortion. The Pope decided that the monk was not guilty of homicide if the fetus was not “
animated.”
Early in the 13th century, Pope Innocent III stated that the soul enters the body of the fetus at the time of “
quickening” - when the woman first feels movement of the fetus. After ensoulment, abortion was equated with murder; before that time, it was a less serious sin, because it terminated only potential human life, not human life.
St. Thomas Aquinas (1225-1274) also considered only the abortion of an “
animated” fetus as murder.
Pope Sixtus V issued a Papal bull
“Effraenatam” in 1588 which threatened those who carried out abortions at
any stage of gestation with excommunication and the death penalty. Pope Gregory XIV revoked the Papal bull shortly after taking office in 1591. He reinstated the “
quickening” test, which he said happened 116 days into pregnancy (16½ weeks).