When does human life begin? This phrasing of this question as it is commonly stated is imprecise and can be misleading. Letâ€™s look at some more precise questions.
When does an individual human being come into existence?
In the biologic sense, this is the point at which a viable human organism first exists, which is the human embryo. Ordinarily, the human embryo comes into existence immediately at fertilization. The human embryo is an organism that can grow and develop, given the proper environment. It is also uniquely human. It will not grow into a rat or an iguana. A human organism is a human entity is a human being.
The fact that some human embryos will die within days does not change the fact that they were human embryos, human beings, for those few days, anymore than the death of a child with a genetic abnormality in the first few days or years of life changes the human beingness of the child up to that point in time.
The fact that some human embryos will spontaneously divide to form twins, or two human beings, doesnâ€™t change the fact that there was one human entity, one human being prior to that time.
When does a woman become pregnant?
As a practical matter in medicine, pregnancy is still defined clinically as beginning actually approximately two weeks prior to conception, as dated from the beginning of the last menstrual period. However, it has been recognized in medicine for many decades that the biological beginning of pregnancy is fertilization, the uniting of the sperm and the ovum.
But around 1973, some medical organizations began defining pregnancy as beginning at implantation, or when the early embryo implants into the inner lining of the womb. (Implantation in the womb normally occurs 5-9 days after fertilization.) This was a deliberately calculated redefinition of the beginning of pregnancy, motivated probably by the development of in vitro fertilization and by wide use of methods of contraception that can act after fertilization, such as the IUD or hormonal methods. But as a distorted view of reality, this definition has many problems. One is that the embryo begins to interact with the mother biochemically and hormonally almost immediately from the moment of conception (fertilization), long before implantation. Another is that there are pregnancies that involve the embryo implanting somewhere outside the womb. These are known as ectopic pregnancies. Most of these occur in the fallopian tubes and most lead after some weeks to the death of the embryo and can be dangerous for the mother. However, some ectopic embryos implant in the abdomen outside the fallopian tubes and uterus and do survive, even to birth (by surgical procedure). Absurdly, these pregnancies and births are never pregnancies at all according to the definition that pregnancy must begin in a motherâ€™s womb. Because of its arbitrariness and logical incoherency, the definition of pregnancy beginning at implantation has never been universally accepted in science. Most embryology textbooks still define pregnancy as beginning at conception (fertilization).
One might object that defining pregnancy as beginning at fertilization has a logical problem in modern medicine. What if fertilization occurs outside the body, as in vitro fertilization? Or what if a human embryo is created through somatic nuclear transfer (cloning) and then transferred to a womanâ€™s body? The answer to this is really quite simple. While ordinarily the woman is first pregnant at the moment of fertilization, in the more general case she is pregnant at the moment that a human embryo enters her body. In the process of in vitro fertilization, at the moment the embryo(s) is (are) transferred to her womb, she is pregnant, whether or not implantation occurs and the pregnancy is maintained.
In summary, although the existence of an embryonic or fetal human organism and the state of pregnancy are closely related, they are not identical. In the future, it may become possible to maintain a human embryo through stages of development that now ordinarily only occur in the womanâ€™s body. (Research in â€œartificial wombsâ€? is currently active.) Pregnancy is the state of having a live human embryo or fetus in oneâ€™s body, whether it enters there in the beginning or sometime later, or whether it is implanted in the womb, somewhere else in the body, or not yet attached.
When does a human person first come into being?
Here is where the crux of the matter lies and where distinctions might be made. When people argue that human life does not begin or exist prior to implantation (or a certain level of fetal development or birth), they cannot logically argue that a developing organism does not exist or that it is not biologically part of the human species. What they can, perhaps, coherently argue is that this organism does not yet have the moral status of a human person. So the questions then are, What is a human person, and when does a human organism become a human person? Many answers have been made to these questions, including viability outside the motherâ€™s womb, the point of the motherâ€™s decision to accept the pregnancy (which implies a possibility of it going out of existence if she changes her mind), certain levels of neurologic development of the fetus, certain levels of self-awareness of the fetus or child. My own opinion is that any definition that subdivides human beings into human persons and human non-persons (or potential persons) is highly morally suspect at best.
Finally, for Latter-day Saints, when does the spirit enter the body?
For Latter-day Saints, who believe in the premortal existence of the spirit, this question is of interest. There is no official Church doctrine on when the spirit enters the body. In the past, a number of prophets and apostles have given various opinions about when the spirit enters the body, including the time of quickening (when the mother feels movement), or just before birth. (Most if not all of these opinions preceded modern understanding of embryology.) My own opinion (purely my own opinion, I emphasize), is that the spirit begins an association with the body from the moment of fertilization. However, itâ€™s important to note that if it is shown that the spirit â€œentersâ€? the body at some point later on, it does not necessarily follow that the human embryo or fetus prior to that point of development has a moral status equivalent to that of a sperm or an egg, which can be treated purely instrumentally, as biological building material. Whether or not the human embryo is a person in a particular sense, it deserves our respect as a human being, an entity that should not be treated as an instrument for the use of others.
Elder Boyd K. Packer stated
We do not know all about when a spirit enters the body but we do know that life, in any form, is very precious. While we are given power to generate life and commanded to do so, we have no license to destroy it.
Elder Boyd K. Packer, “The Fountain of Life,” BYU 18-Stake Fireside, 29 March 1992, p. 4
In the 1995 Proclamation on the Family, the First Presidency and Quorum of the Twelve state:
We declare the means by which mortal life is created to be divinely appointed. We affirm the sanctity of life and of its importance in Godâ€™s eternal plan.
I conclude that mortal human life is a sacred trust from God from its beginning through natural death, regardless of when the spirit enters the body. Based on these considerations, I have made the personal decision as a physician that I will do nothing to deliberately interrupt the processes of human development at any of its stages.