I think the way GP phrased it is a shorthand for "egg just after fertilization" - because today, fertilized eggs either get implanted early or rejected by the mother's body. When science and technology advances to the point of making it possible to gestate humans entirely outside of the womb, the way we talk about this will have to adjust to be more precise.

(Implantation itself doesn't feel like the transition point either, but it's just the last obvious discrete step before the continuous progression all the way to birth.)