"sperm-egg union is a necessary condition for conception, but not a sufficient condition."

...Do such things as hydatidiform moles, choriocarcinoma, blighted ovum, and clones prove that human life does NOT begin at conception?

There is a second argument for implantation as the decisive moment: If we say that full humanness begins at conception, we must respond to the observation that "some entities that stem from the union of sperm and egg are not 'human beings' and never will develop into them," and that there may be some human beings who come into being without the union of sperm and egg.[4]

Nathanson gives examples of nonhuman entities that result from the sperm-egg union:

• hydatidiform mole ("an entity which is usually just a degenerated placenta and typically has a random number of chromosomes")
• choriocarcinoma ("a 'conception-cancer' resulting from the sperm-egg union is one of gynecology's most malignant tumors")
• blighted ovum ("a conception with the forty-six chromosomes but which is only a placenta, lacks an embryonic plate, and is always aborted naturally after implantation").

A clone is an example of a human entity that may come into being without benefit of a sperm-egg union.[5]

The problem with Nathanson's argument is that he confuses necessary and sufficient conditions. One who holds that full humanness begins at conception is not arguing that everything which results from the sperm-egg union is necessarily a conception. That is, every conception of a unique individual human entity is the result of a sperm-egg union, but not every sperm-egg union results in such a conception. Hence, the sperm-egg union is a necessarycondition for conception, but not a sufficient condition.

Furthermore, Nathanson is correct in asserting that it is possible that some day there may be human beings, such as clones, who come into existence without benefit of conception.

[6] But this would only mean that conception is not a necessarycondition for full humanness, just as the sperm-egg union is not asufficient condition for conception.

In sum, Nathanson's argument from both nonhuman products of sperm-egg unions and the possibility of clones is inadequate in overturning the pro-life position that full humanness begins at conception.

Spontaneous abortions.

It is estimated that twenty to fifty percent of all conceptions die before birth. Thirty percent, it is estimated, die before implantation.

[7] Some people argue that these facts make it difficult to believe that the unborn are fully human in at least the very earliest stage of their development prior to implantation. But this is clearly an invalid argument, for it does not logically follow from the number of unborn entities who die that these entities are not by nature fully human.

Suppose the pro-choice advocate responds to this by arguing that if every fertilized ovum is human, then we are obligated to save all spontaneous abortions as well. But if we did, it would lead to overpopulation, death by medical neglect, and starvation. The problem with this response is that it confuses our obvious prima facie moral obligation not to commit homicide (that is, to perform an abortion) with the questionable moral obligation to interfere with natural death (that is, to permit the conceptus to abort spontaneously).

Protecting life is a moral obligation, but resisting natural death is not necessarily a moral duty...There is no inconsistency between preserving natural life, opposing artificial abortion and allowing natural death by spontaneous abortion.[8]

Admittedly, the question of interference in spontaneous abortions provokes the pro-life ethicist to think more deeply and sensitively about his or her position and to make distinctions and nuances that may not be pleasing to all who call themselves pro-life. But just as the difficult question of whether to pull the plug on the irreversibly comatose who are machine-dependent does not count against the position that murdering healthy adults is morally wrong, the question of how we should ethically respond to spontaneous abortions does not count against the pro-life ethic which says that we should notdirectly kill the healthy and normally developing unborn.

Twinning and recombination.

Some people argue that since both twinning (the division of a single conceptus) and recombination (the reuniting of two concepti) occur prior to implantation, individual human life does not begin until that time. However, a careful examination of the nature of twinning and recombination reveals that there is no reason to suppose that the original pre-twinned conceptus or any pre-recombined conceptus was not fully human.

First, scientists are not agreed on many aspects of twinning.

• Some claim that twinning may be a nonsexual form of parthenogenesis or “parenting.” This occurs in some animals and plants.
• Others claim that when twinning occurs, an existing human being dies and gives life to two new and identical human beings like himself or herself.
  
• Still others claim that since not all human concepti have the capacity to twin, one could argue that there exists in some concepti a basic duality prior to the split. Hence, it may be claimed that at least in some incipient form two individual lives were present from the start at conception.

In any event, the fact of twinning does not seem to be a sufficient reason to give up the belief that full humanness begins at conception.[9]

Second, every conceptus, whether before twinning or recombination, is still a genetically unique individual who is distinct from his or her parents.

In other words, if identical twins result from a conceptus split or one individual results from two concepti that recombine, it does not logically follow that any of the concepti prior to twinning or recombining were not human.[10]

To help us understand this point, philosopher Robert Wennberg provides the following story:

Imagine that we lived in a world in which a certain small percentage of teenagers replicated themselves by some mysterious natural means, splitting in two upon reaching their sixteenth birthday. We would not in the least be inclined to conclude that no human being could therefore be considered a person prior to becoming sixteen years of age; nor would we conclude that life could be taken with greater impunity prior to replication than afterward. The real oddity—to press the parallel—would be two teenagers becoming one.

However, in all of this we still would not judge the individual's claim to life to be undermined in any way. We might puzzle over questions of personal identity... but we would not allow these strange replications and fusions to influence our thinking about an individual's right to life. Nor therefore does it seem that such considerations are relevant in determining the point at which an individual might assume a right to life in utero." [11]

From [ChristianAnswers.net]