The question of research on embryos did not arise before the late 20th century. The moral status of the early embryo in the Christian tradition has therefore to be deduced from attitudes to abortion.

In the Christian tradition abortion at any stage has always been regarded as gravely sinful. However, for many centuries the termination of a pregnancy at an early stage carried lesser penalties than one later. This was related to the view that the human soul did not enter the embryo ("ensoulment") until 40 days after conception in the case of a man, and 90 days after conception in the case of a woman, an understanding that was taken over from Aristotle. This distinction in the seriousness of early and late abortion was grounded in the Greek and early Latin translations of Exodus 21, 22 which drew a distinction between the formed and unformed foetus.

These distinctions became the established position from at least the 12th to the 17th centuries and are present in significant Western writings of earlier periods. The evidence of the earliest Christian centuries is open to different interpretations on this issue. The Eastern Church, however, does not acknowledge such gradations in the seriousness of abortion.

In 1869 Pope Pius IX declared that all mothers who had survived an abortion were to be excommunicated making no reference to the earlier distinction between animate and un-animate foetuses and implying that a person was ensouled from conception onwards. For many Christians today, not just Roman Catholics, this position is definitive because, with the outmoding of the Aristotelian concept of delayed ensoulment, fertilisation is the point at which human life emerges and, as vulnerable human life, it is particularly worthy of protection. Many theologians take the view that, because the early embryo may be a person and because this is such a crucial matter, the embryo must be given the benefit of any doubt and there is therefore a moral obligation to offer all the protection that would be accorded a baby or adult.

For other Christians, however, the fact that the Christian tradition, for so much of its history, made a distinction between the moral status of the unformed and the formed embryo, and thought of the human person in the full sense coming only with a delayed ensoulment, remains significant: it reflects a valid moral distinction which needs to be affirmed even with the outmoding of the Aristotelian philosophy on which it was once based.