Sex assignment
Even though it is called "assignment," these adults are not literally assigning (choosing) whether the child is male or female.  In most people, the sex assigned at birth matches the person's own gender identity. If the assigned sex does not match a person's gender identity, the person is transgender.  In a very small number of newborn babies, it is not possible to tell whether the child's body is male or female by looking at the sex organs.  These people are intersex.