For the last half-century, the Catholic Church has actively taught that life begins at conception. They did not always advance that argument. In the middle ages, priests developed texts known as "penitentials" that were designed as guides in the confessional. They provided standard penances for a wide variety of confessed sins. The penitentials provided different punishments for women who had an abortion depending upon whether it was before or after "the quickening" (the point at which fetal movement can be felt by the mother). If it occurred after the quickening, the woman was assigned far longer penance than for an early abortion. This seems to indicate that, earlier in the history of the church, life was considered to begin at the quickening, not at conception.
Jewish tradition teaches that the soul enters the child at birth. Even today, many Orthodox Jews will not address an unborn child by name or even set up a nursery in anticipation.
The tricky thing about Catholic Church doctrine is that, according to the Church, it does not change over time. When a new doctrine is developed in response to a change in society, the Church claims to be simply clarifying and articulating something that was always true. For many centuries, there was no need to worry about the Virgin Mary being a carrier of original sin, because the mother was believed to be just the vessel for the father's seed. She may shape her offspring, as a plant's roots grow in the shape of the pot, but she does not pass on her nature. Gregor Mendel's studies of genetic inheritance, however, made people aware that Jesus must have inherited genes from his mother. Thus, the Catholic Church developed the doctrine of the Immaculate Conception, whereby Mary was the product of a specially-blessed union and was miraculously conceived without inheriting original sin from her parents.
When birth control pills became available, women had the ability to control their reproductive destiny far more effectively than ever before. The pills prevented pregnancy before it ever happened by suppressing ovulation and possibly also by preventing implantation if a woman did ovulate. Since medical science recognizes pregnancy as beginning at implantation (since a great many fertilized eggs are naturally expelled by the body for a variety of reasons), conventional birth control pills do not end a pregnancy.
It was after the pill became available that the Catholic Church began teaching that life begins at conception, not implantation nor the quickening. Many Protestant churches followed their lead. This doctrine is logically problematic for a couple of reasons.
First, the Church condemns hormonal birth control for the stated reason that it can prevent a fertilized egg (which the Church now says is a human life) from implanting in the womb, and thus the pill kills babies. However, the Church encourages breast feeding, even though lactation causes hormonal changes similar to the pill and also can prevent a fertilized egg from implanting in the womb. Why does breast-feeding get a pass when birth control pills are condemned? If the Church is more concerned with making a woman's body subordinate to a baby than with saving the lives of fertilized eggs, the difference makes sense.
Secondly, prior to implantation (and even for a short while afterward), it is still undetermined whether that fertilized egg will grow into one baby or two or three (or even five, in the case of the famous Dionne quintuplets). If the soul enters at conception, does it split when the zygote does? Do twins each possess only half a soul? Also, twins sometimes merge during the course of a pregnancy. This even happens with fraternal twins, who grew from two separate fertilized eggs. Some people are born with different DNA in different body parts as a result of such in-utero mergers. Do these people have two souls?
My faith does not require me to believe that life begins at conception, and my knowledge of science and of church history does not allow me to believe it. Therefore, in my opinion, any legal restrictions on first trimester abortion are based entirely on the desire of some people to impose their religious beliefs on others.
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