"Casti Connubii," a papal encyclical given by Pope Pius XI on 31 December 1930, served primarily as a reaffirmation and expansion of the issues discussed in Arcanum, an encyclical written by Pope Leo XIII. It was released to address new threats to marriage and conjugal unity, and indeed is translated "On Christian Marriage" or "On Chastity in Marriage." The document explores the meaning of Christian marriage and emphasizes its threefold purpose as borrowed from St. Augustine: to produce offspring, to grow in conjugal faith, and to show benefit from the sacrament. It begins by exploring the nature of marriage, followed by a discussion of its advantages for individuals and societies, erroneous but common beliefs about marriage, threats to pure marriage, and finally how to address them. Included in the threats to pure marriage is that of the growing popularity of contraception and abortive procedures, at which point Pope Pius XI elaborates on the Church' s statement that life begins at conception.

Widely known as a key contributor to the Roman Catholic Church's body of doctrine, St. Thomas Aquinas also published an opinion on the moral status of embryos and fetuses that seems contradictory to the Catholic Church's current standpoint on the matter. Born in Naples, Italy, around 1225 (scholars debate the exact year of many of his life events) to wealthy nobility, Thomas Aquinas quickly proved himself a pious and astute scholar with an insatiable desire for logic and understanding. After receiving his formative education in Montecassino and Naples, Italy, Aquinas joined the order of the Dominicans. His desire for the holy life shocked and upset his family, who lamented his choice of a poor lifestyle devoted to service. To prevent Aquinas from following through with his plan, his family held him captive in the San Giovanni fortress in Rocca Secca, Italy, for nearly two years. After his mother and siblings noted his devotion to the Church evidenced by his daily studies and constant writing (not to mention his dismissal of concubines), they relented and allowed him to take his vows with the Dominicans around 1245. It is also estimated that he officially received his Master's in the Arts from the University of Naples around this time.

Mother Teresa, a Roman Catholic nun known for her charitable work and attention to the poor, was born 26 August 1910. The youngest child of Albanian parents Nikola and Drane Bojaxhiu, she was christened Agnes Gonxha Bojaxhiu and spent her early life in the place of her birth, present-day Skopje, in the Republic of Macedonia. In addition to her unwavering devotion to serve the sick and the poor, Mother Teresa firmly defended traditional Catholic teachings on more controversial issues, such as contraception and abortion. Indeed, her addresses to Western nations rarely excluded straightforward commentary on the social circumstances and disagreements taking place there. Mother Teresa, a renowned humanitarian, defended her religious beliefs regarding life issues and based her life on serving unwanted people in societies around the world until her death in 1997.

Pope John Paul II's views on abortion and embryology have been very influential to the Roman Catholic Church. He strictly forbade abortion and other threats to what he regarded as early human life in his encyclical entitled "Evangelium Vitae," meaning the "Gospel of Life." His authority on moral and social issues was highly regarded during his lifetime. Known around the world for his peace efforts and extensive writings on everything from poverty and Communism to contraception and reproductive issues, Pope John Paul II's legacy consists of his major contributions to the Roman Catholic Church' s body of doctrine and the effects of his long period of influence on Catholics and governments throughout the world.

Mizuko Kuyo is a Japanese Buddhist ceremony that focuses on a deceased fetus or stillborn child. This ceremony was originally developed to honor Jizo, a god believed to be responsible for transporting dead fetuses or children to the other world. The practice has become more popular in the last half century due to the growing number of abortions taking place and the particular views that Japanese Buddhists have about fetuses and abortion. Japanese Buddhists believe that honoring Jizo will ensure that their aborted fetus successfully makes it to the other world, where it can be reborn in the future. In a religious context, Mizuko Kuyo provides an example of one of the many ways that fetuses are viewed and treated both in natural death and in abortion.

Wilhelm August Oscar Hertwig contributed to embryology through his studies of cells in development and his discovery that only one spermatozoon is necessary to fertilize an egg. He was born 21 April 1849 to Elise Trapp and Carl Hertwig in Hessen, Germany. After his brother Richard was born the family moved to Muhlhausen in Thuringen where the boys were educated. The two brothers later attended the university in Jena from 1868 to 1888 and studied under Ernst Haeckel, who later convinced Hertwig to leave chemistry and pursue medicine. Hertwig became an assistant professor of anatomy at Jena in 1878 and full professor three years later. He was the first chair of both cytology and embryology in Berlin from 1888 to 1921 and director of the new Anatomical-Biological Institute there. Hertwig also became a member of the Prussian Academy of Sciences in Berlin and the Leopoldina Academy in Jena.

Known for dropping a long-held distinction in the Catholic Church between the animated and unanimated fetus, Felice Peretti was born in Grottamare, Italy, in 1521, son of a Dalmatian gardener. In his early years, Peretti worked as a swineherd, but soon became involved in the local Minorite convent in Montalto, where he served as a novice at the age of twelve. He went on to study in Montalto, Ferrara, and Bologna, continuing his devotion to religious life, and in 1547 Peretti was ordained as priest in the city of Siena.

Pope Pius XII was born Eugenio Maria Giuseppi Giovanni Pacelli on 2 March 1876 in Rome, Italy, to Virginia and Filippo Pacelli. Known for his oft-disputed role in the Roman Catholic Church's approach to the Nazis and World War II, Pope Pius XII also contributed a number of important documents regarding conception, fertility, abortion, and reproductive control to the Vatican's collection of writings and doctrine on procreation.

Quickening, the point at which a pregnant woman can first feel the movements of the growing embryo or fetus, has long been considered a pivotal moment in pregnancy. Over time, this experience has been used in a variety of contexts, ranging from representing the point of ensoulment to determining whether an abortion was legal to indicating the gender of the unborn baby; philosophy, theology, and law all address the idea of quickening in detail. Beginning with Aristotle, quickening divided the developmental stages of embryo and fetus. Indeed, this concept influenced the way embryology, human development, and hominization have been understood for over two thousand years.

As various societies around the world began legalizing abortive procedures or liberalizing government stances on abortion, the Roman Catholic Church's leaders felt the need to respond to these changes by clarifying the Church's position on procured abortion. One incident in particular that may have inspired the "Declaration on Procured Abortion" is the landmark case in the United States Supreme Court in 1973: Roe v. Wade. This case, which legalized a wide range of abortion procedures in perhaps the most influential nation in the world, spurred conversation regarding the morality of procured abortion throughout the world. The Vatican's response was initiated by the Sacred Congregation for the Doctrine of the Faith which, under the guidance of the Prefect Franciscus Seper and Archbishop Hieronymus Hamer, became the "Declaration on Procured Abortion."