From what I have learned about human development, fertilization is the most logical point to say "an individual human life has started." I do not draw a distinctino based on whether this occurs in vitro or in vivo, with a penis or with a turkey baster. From then on, it's a steady progression through normal development, if in the proper environment. I think it is silly to say that one embryo is less human if it is in a dish rather than a womb. I think that's as silly as saying a two-month old left outside on a cold winter night is less human than one tucked into a crib because one lived and one died. To me, those are arbitrary, irrational demarcations. I think environment, size, whether she's "wanted", and many other criteria are arbitrary and irrational to use to define a human being. I find people weak and emotion-driven when they use genes to select which kid to have.