Yes, thank you, but not an equivalent blob. A newly fertilized egg, for one thing, doesn’t post opinions on the Internet. While it’s possible that I may have at some point been criticized as “a shapeless amorphous blob with no brain”, I assure you that such a comment would have been meant figuratively and not literally! 
“Respond” how? Plants respond to light, and even the simplest organisms respond to various primitive stimuli. Much of this is the same kind of imaginary thinking that led Terry Schiavo’s parents and “pro-life supporters” to imagine that she was responding to being spoken to, or having her hand held. Until the autopsy after her death showed that her brain had atrophied to a non-functional condition. The emotional projection by those who loved her was understandable, and I don’t mean to sound cruel, but it was still purely imaginary wishful thinking.
There is no point at which any identifiable “magical” thing happens, but birth is the point that the baby takes its first breath and becomes incontrovertibly a separate being independent of its mother’s body. It’s usually a legal demarcation point only because the law sometimes has to draw these lines in order to establish clear policy.
I know it’s a serious question. It’s the question at the heart of the abortion debate. Most rational, evidence-based attempts to answer it center on the question of higher-level brain function and perhaps sentience, although the former remains interpretively subjective and the latter perhaps impossible to assess. The issue isn’t what I personally believe because someone else will believe something different, and no amount of technical expertise will resolve it, although it can help to inform the moral judgments. The issue is what to do when such a question cannot be morally, philosophically, and scientifically resolved to some definite stage of development. The anti-abortionists declare that the consequence should be that abortions should never be allowed at any stage, for the protection of God’s miracle, or something, even to the point of prohibiting abortions at a ridiculously early stage. Whereas the pro-choice argument is characterized by its name: with no possible objective resolution, keep legal dictum out of it, because dogmatically motivated irrationality on this matter has real and direct human cost on real live human beings.
I understand the emotion around abortion, I really do. I understand the emotion around a miscarriage, even at a very early stage where many actually go unnoticed. “What might have been” is emotionally powerful – my use of the phrase comes from John Greenleaf Whittier, “For all sad words of tongue and pen, the saddest are these ‘It might have been’.” Very true, and very moving. But as a basis for abrogating the real rights of real people, it’s entirely emotional and/or dogmatically religious, and in no way rational. And it’s the only argument the anti-abortionists have.