Scientifically, however, a fertilized egg is the first point in the process where you have a new individual. That's a rather solid line to use, even if it is rather inconvenient for certain purposes. Of course, depending you your point of view, that may be a benefit of the line, not a problem.
A lot of ethical considerations stem from what you consider to be a "human". While you can set that point anywhere you want to, the problem is also that you can set that point anywhere you want to. With the ability to genetically engineer humans, it's far too convenient to state that they're not human until you're done altering their genome at the most obvious point of intervention.
I think it's a mind that defines a person, that a mind depends on neuron activity in the cerebral cortex, and that this activity, as evidenced by brain waves, commences after 20 weeks gestation (18 weeks of pregnancy). This seems far more relevant to me than when there is an individual body. Also, twinning can occur, or a fertilised egg may produce only a placenta, so I don't think it's accurate to call fertilisation "the first point in the process where you have a new individual" even if you are only interested in bodies.