In regards to human reproduction, both the egg and sperm cell are alive. The combination of egg and sperm produces a zygote which initiates the processes of biological development. The extent to which that zygote is considered a person or a potential person depends on the beholder. In other words, the bounds of individual personhood are yet to be uniformly agreed upon, and people (and religions) have put forth varied interpretations. .

The Roman Catholic Church provides one of the strongest positions in opposition to the use of embryos for research purposes. Catholic belief holds that life is sacred from the moment of fertilization and that the embryo is the inception of personhood, no matter where it exists. The church maintains that every embryo should be given the opportunity to develop into a mature human being.

Protestantism ranges from one extreme to the other on the issue of embryonic stem cell research. Several denominations, such as the Southern Baptist Convention, hold that the embryo is the tiniest form of human life and should not be destroyed, while others, such as including the American Presbyterian Church, maintain that the research is acceptable if the goals cannot be reached in any other manner. The majority of Protestants in favor of stem cell research also hold that research must be limited to embryos that cannot be used for reproductive purposes, within a 15-day window from fertilization.

Jewish biblical and Talmudic law holds that human status as a person is acquired progressively, not at fertilization. Jewish traditions do not give moral, human status to embryos until they have reached 40 days of gestation. The fetus only achieves personhood at birth. Following these views, Judaism supports ethically justifiable research using embryos for research and therapeutic purposes.

Following Islamic beliefs in the Quran, the use of embryos for research and therapeutic purposes is acceptable from fertilization through the 40th day of development. Most modern Muslims agree on a moment beyond the blastocyst stage that marks when a fetus becomes a human being. Because of the belief that personhood is a developmental process, the majority of Sunni and Shiite jurists support ethically regulated embryonic stem cell research.

In traditional Hindu belief, conception is the beginning of a soul's rebirth from a previous life. Some Hindu traditions place the beginning of personhood between three and five months of gestation, while few believe that incarnation can occur as late as the seventh month. Most Buddhists have adopted the classical Hindu teaching that personhood begins at conception.

Most individuals with a naturalistic worldview would likely value embryonic stem cell research. They would assert that the good that can be produced by pursuing such research (e.g., medical advances) outweighs negative aspects. Regarding when personhood begins: life is viewed as a biological process of gradual development. Some would reject use of embryos for research past some point in the development, whereas some would take a position more in line with the Judaic position and say that full human status and civic status come only at birth. In general no personhood would be seen to precede sentience.

The above summaries on religions are adapted from Science & Theology News, Dec. 2004 - Jennifer Cousins. Paul Geisert contributed the discussion on personhood and the naturalistic worldview.   May  2005