Hello! Due to a request to make the answer to your question “re-bloggable,” the full response to your question has been moved here!
Here is a summary:
The question, “When does life begin?” is scientifically, an arbitrary question in itself – any satisfying answer would have to be philosophical in nature. Based on current biology, there is no singular definition of life - Life is more of a characteristic. The progression of a zygote into a human being is a developmental process, and there is no one point in development where we can label a blastocyst or embryo, or even fetus as an actual human being. What we can do, however, is use tangible markers such as when a fetus’ heart starts beating, or its nervous system begins to develop, or it first begins responding to external stimuli to really get a sense of what the life form really is.