Don't you agree that this statement is true?
Oh. Ok. So we're back to the unremarkable observation that human reproduction and development involves a lot of "stages." I am, of course, happy to agree with that, but I don't find it any more insightful than an observation that weather involves a lot of "patterns."
The difference, of course, is that some folks are eager to assign different values to the different stages of human life in order to promote a utilitarian argument. By acknowledging that a blastocyst is a certain stage of human development, one acknowledges the possibility that it might have intrinsic value as human life. This is an important point, when considering a utilitarian argument.
One can then go on to argue about the intrinsic value of this particular form of human life, if one wants to. I, however, am of the opinion that this value is ultimately unknowable, and thus must be divorced from utilitarian analysis.
To extend it to your weather pattern analogy, if somebody declared all rainy days of little worth, and got hold of Karl Rove's Weather Machine, and set about eliminating rainy days because he found sunny days to be of higher value, then we're getting into the same territory. The counter argument that a rainy day weather pattern may be as valuable as a sunny day weather pattern would start with the acknowledgement that they are both weather patterns, and both connected to a larger whole, and may both have similar intrinsic value.
If one just says that rainy days are not sunny days and that sunny days are better, one misses the point.