>> How can you have a universal definition of "when life starts" (rather then a general definition of life, in case of abortion) >>
If you want to get technical and non-religious, life never starts.....it only ends and continues.
One anti-conception argument is that the egg is capable of splitting after conception and making paternal twins. Clearly prior to the egg splitting and make two new humans the egg must have been something else as that form of reproduction just doesn't exist in what any rational person would call human.
I would say it is alive and human at some point whenever the fertilized egg can no longer reproduce itself but would die if split. JMO.