Sometimes the issue of abortion is supported by arguments that say human life does not begin at the moment of conception. Instead, it is argued that the human life in the womb is a "clump of cells" that can be destroyed.
Yet, these arguments based on the "science" of conception are simply false, and do not lend any support for abortion.
Genetic science
The Sacred Congregation for the Doctrine of the Faith pointed this out already in 1974, in their Declaration on Procured Abortion.
Primitive societies could theoretically make such an argument, not knowing exactly what happens in the womb. However, modern science is clear that everything is present at the moment of conception to produce a unique human person.
Science supports protecting life
St. John Paul II pointed this out in his encyclical Evangelium Vitae, explaining that in reality science supports the case of protecting human life in the womb.
Last of all, the Vatican in its document stated that the topic of abortion should not be based on biology, as it falls squarely on human morality.
The fact of the matter is that abortion is about killing another human person. Anyone who supports abortion has to come to terms with that reality and ask themselves if society should allow the deliberate killing of another human.