This type of answer is profoundly unsatisfying, but it’s about all you can expect if you ask the wrong question. - Harold J. Morowitz

The net result is that slightly fewer than a third of all conceptions lead to a fetus that has a chance of developing. In other words, if you were to choose a zygote at random and follow it through the first week of development, the chances are less than one in three that it would still be there at full term, even though there has been no human intervention. Nature, it seems, performs abortions at a much higher rate than any human society. It is simply not true that most zygotes, if undisturbed, will produce a human being.

The end point of this reasoning is that any policy based on assigning a unique status to conception in the emergence of humanness must be seen as coming from subjective evaluations—evaluations that may not be shared by others. Subjectivity does not, of course, does not make these arguments wrong; it simply means that they cannot be given the kind of public universality we assign to arguments grounded in scientific understanding.

At the chemical level, human beings just aren’t all that different from pumpkins or any other life forms.