There are several stages of development that could be given increasing moral status:
1. Implantation of the embryo into the uterus wall around six days after fertilization.
2. Appearance of the primitive streak – the beginnings of the nervous system – at around 14 days.
3. The phase when the baby could survive if born prematurely.
4. Birth.
If a life is lost, we tend to feel differently about it depending on the stage of the lost life. A fertilized egg before implantation in the uterus could be granted a lesser degree of respect than a human fetus or a born baby.
More than half of all fertilized eggs are lost due to natural causes. If the natural process involves such loss, then using some embryos in stem cell research should not worry us either.