Legal Death, Legal Life

This would in effect make the point of "uniform life" to be the earliest possible point at which assisted premature birth would be medically feasible without causing significant effect on the end result.

Letting the families concerned make the decisions on what to do with the surplus embryos meets the needs for a plurality of beliefs to be accommodated within our society, in the same way that it is for terminally ill patients, and it meets the same ethical demands of each of those families.

I see a problem with it, though. It's this: as medical science progresses, the point at which assisted premature birth is medically feasible will move back further and further until a point is reached where it is medically feasible to fertilize an egg outside the womb and keep it there, in an artificial incubator. It may become possible to let a fertilized egg grow into a baby without the need for it ever to be born. In that case, the fertilized egg would constitute a viable human being to which the definition applies.