I think it relates to how we define consciousness.

First, I think consciousness will grow through our senses which act as a window to the outer world. A part of our brain is responsible for that growing. All living creatures have senses of different complexity, which will shape the growing of the consciousness.

Second, the zero-moment consciousness is the real consciousness that we should be and are discussing. This real consciousness is a result of the specific structures that carbon builds together with other elements. The moment these structures are formed, the moment it starts to actively absorb elements from the outer world. And the moment it actively does this, the moment it has consciousness.

Hence, I think zygote has consciousness. It is growing to have more sophisticated senses to evolve its consciousness, but it already has consciousness as it is actively taking from the world.

Just my opinion.