The problem is that because development is so continuous and unbroken, there is no obvious point in between that could serve as the dividing line required by the binary division; any proposed point would be altogether arbitrary.
Exactly. Even if one wishes to use consciousness as a criterion, there can be some discussion. Barring other anomalies that may negatively impact on the quality of life for the organism, at what level of awareness does it become ethically objectionable for us to end that lifeform's development?
The CNS forms relatively early on in the first trimester. So it gets very difficult fairly quickly.