It is a zygote.
It is also a human being (living human).
It is also a child (offspring).
It is not a baby. (meaning a newborn, infant, and/or toddler)
It is not a fetus.
Zygote, embryo, fetus, newborn, infant, and toddler are all particular stages of human life and development.
A human being is a human being, regardless of the particular stage of human life and development that he/she is in.
One's child is one's child, regardless of the particular stage of human life and development that the child is in.