Is the abortion of a fetus due to premature birth considered an expiation for past masturbation?

You are not liable for any sin, expiation, or blood money due to the premature birth and the child's death, because there was no direct or certain cause from your part that led to it, but merely conjectures. The default is blamelessness, and one does not deviate from this default except with certainty. The child's death was by Allah's permission and will, so there is no need for worry or feelings of guilt.