Below is what I found on the internet about the issue.
On another catholic forum thread it is discussed as to why the Church needed “scientific knowledge” to make “infallible” papal proclimations on the faith and morals issue of when life begins. Many Catholics would think that God would somehow would intervien and guide the Church without human “scientific knowledge” on issues of faith and morals.
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forums.catholic-questions.org/showthread.php?t=38663
********The Roman Catholic Church and Abortion: An Historical Perspective **
It is of interest that the Eastern (Orthodox) Fathers did not subscribe to the pro-abortion Western (Catholic) teaching which was based on the belief that a foetus “quickened” and became alive roughly 17 weeks after conception… The Eastern Fathers knew of this Western argumentation but they dismissed it and insisted that a foetus was human and “quickened” from the first moment of conception.
St. Augustine (AD 354-430) said, “There cannot yet be said to be a live soul in a body that lacks sensation”, and held that abortion required penance only for the sexual aspect of the sin.
He and other early Christian theologians believed, as had Aristotle centuries before, that “animation”, or the coming alive of the fetus, occurred forty days after conception for a boy and eighty days after conception for a girl. The conclusion that early abortion is not homicide is contained in the first authoritative collection of canon law accepted by the [Catholic] church in 1140. As this collection was used as an instruction manual for priests until the new Code of Canon Law of 1917, its view of abortion has had great influence.
At the beginning of the 13th century, Pope Innocent III wrote that “quickening” “the time when a woman first feels the fetus move within her” was the moment at which abortion became homicide; prior to quickening, abortion was a less serious sin.
Pope Gregory XIV agreed, designating quickening as occurring after a period of 116 days (about 17 weeks). His declaration in 1591 that early abortion was not grounds for excommunication continued to be the abortion policy of the Catholic Church until 1869.
The tolerant approach to abortion which had prevailed in the Roman Catholic Church for centuries ended at the end of the nineteenth century. In 1869, Pope Pius IX officially eliminated the Catholic distinction between an animated and a nonanimated fetus and required excommunication for abortions at any stage of pregnancy.
The Orthodox Churches of the East never went through any period when abortion was considered less than a very grave sin, right from the moment of conception.
“A woman who deliberately destroys a fetus is answerable for murder. And any fine distinction as to its being completely formed or unformed is not admissible among us.”
St. Basil the Great, Three Canonical Letters
This was then incorporated into the Canons of an Ecumenical Council, and additionally:
The 91st canon of the Quinisext Ecumenical Council (691 A.D.):
“Those who furnish drugs for the purpose of procuring abortion, and those who take fetus-killing poisons are subject to the penalty prescribed for murderers.”
Quoted from
catholicculture.org/docs…cfm?recnum=3361