Conception starts a new life. Birth brings a new human being into the world. Somewhere between these two events, a new person comes into existence. When does a developing fetus attain personhood the property of being a real person? Although a direct progression from fertilized egg to embryo to fetus to baby represents the normal flow of events, it frequently does not hold true.
A fertilized egg may simply fail to implant in the uterus. In this case no pregnancy occurs and the egg is simply washed out of the body with the normal menstrual flow. No churches advocate examining a menstrual flow to identify such unfortunate eggs so that they could be baptized and given a proper funeral. A fertilized egg may also implant outside the uterus, particularly in a Fallopian tube. Then the egg immediately loses any potential to become a human being and is instead transformed into a growth threatening the woman's life. And at any stage of a pregnancy, a woman's body may spontaneously abort the developing embryo or fetus, often because of major defects. If this happens at the earliest stages, the woman may even be unaware that a pregnancy occurred. But a miscarriage at a very late stage my well result in baptism and a funeral. Defective fertilized eggs can also result in molar pregnancies in which a nonviable embryo implants and proliferates within the uterus.
A single fertilized egg can also divide into two (or more) separate clusters, which then develop into identical siblings. The opposite case is also possible, with two fertilized eggs developing into a single person. Scientists have created mice with four or more parents by taking the developing cell clusters from multiple fertilized eggs and combining them into a single cell cluster which then develops into a single individual. Such seemingly normal mice are genetic mosaics; some cells in their bodies are from one parental lineage and some from another. The same thing can spontaneously happen in humans, with the developing cell clusters from two separate fraternal twins combining and developing into a single person. Such an individual could go though life totally aware of his or her unusual genetic makeup, or some unusual circumstance could bring it to light. This was the case with one Florida woman who gave birth to a child with an incompatible blood type. Looking into this strange situation, doctors discovered that she was indeed a mosaic of two different genetic lineages. Biologically, she was her own twin - two persons wrapped into one.
Other anomalous situations can arise with twins. The developing embryo from one fraternal twin can envelop and effectively encapsulate the other developing embryo. In this case, the healthy twin will be born perfectly normally and may never become aware of the stunted embryo hidden somewhere within his or her body. Or surgical removal of a bothersome cyst or tumor later in life may disclose the remnants of a lost twin.
Medical history is replete with cases where the clusters of cells from developing identical siblings did not fully separate, or the clusters from fraternal twins partially fused. This can result in conjoined twins, who may or may not be able to be separated to live individual existences. It may also result in a wide variety of bizarre outcomes. Krista and Tatiana Hogan are twins, connected at their heads but displaying two personalities. Similarly, Abigail and Brittany Hensel are clearly two persons with one body and not one person with two heads. Presumably, they could marry separate husbands. In other highly publicized recent cases, one girl born in India had one head but two faces; another had eight limbs. Surgery rearranged some of her organs and removed the excess limbs, which were attributed to a parasitic twin. In individual cases, it may be difficult to decide whether there is one person or two.
Human beings all begin as a fertilized egg. But there are multiple ways a fertilized egg can lose, or expand, its potential to become a person. There is no simple correlation between fertilized eggs and human beings. From a theological or moral point of view, a new person emerges when God infuses an immortal soul. The question of exactly when this occurs is something that biology cannot answer. But it is a question that legal systems have to address, deciding which actions to criminalize and which not.
A legal system is ultimately grounded in the moral values of its citizens, but rarely does everyone agree on just what is moral or criminal. Killing is universally recognized as immoral and criminalized, though exceptions are also universally provided, including self defense and legal executions. Penalties for suicide are obviously unenforceable, though assisting in a suicide is generally illegal. Various sexual acts widely considered immoral were long criminalized, but most all of these criminal sanctions have been lifted. Child pornography remains widely considered immoral and criminalized, as are public sex acts. Generally, in civil society, sins are not criminalized unless they have a clear civil impact murder does but private sex acts generally do not. Morals (and civil law) also change over time. Slavery was long considered moral and legal, while miscegenation was long considered both immoral and criminal. Blue laws criminalizing various activities on Sunday were initially based on religious practices; some still survive now justified as a means of providing a community some quiet periods.
In the United States it is not unusual for laws to make accommodation for religious beliefs: military persons can wear turbans, conscientious objectors can perform some alternative to military service, Federal employees get a reasonable allowance for religious holidays, etc. These exceptions are granted to religious persons, but are not made mandatory for any one else.
In regards to personhood, considering conception as creating a new person provides a satisfying black and white answer, but avoids the key question of when does God infuse a soul. Even churches that theoretically recognize very early stages as persons, do not recognize them in practice: baptism, funeral services and religious burial for fertilized eggs and embryos is at the very least highly unusual. The conclusion is unavoidable that such a determination of personhood is a religious judgement that is not shared by the majority of citizens. As such, it is simply not a legitimate basis for legal determinations.
Whether or not recognized as a separate person, it still makes sense to criminalize the willful destruction of a viable embryo by a third person. The embryo can be a precious possession of the expectant mother. Destroying it can have immediate and negative consequences for the expectant mother and her family. We do not have to accept an embryo as a person to recognize it as a protected living creature; indeed, a very special creature for it has a potential to become a human being. But having the potential to become a human being does not make it a person.
This post has been modified since its original publication.