The Biblical word for miscarriage, untimely birth, or abortion is undifferentiated into these separate categories as we would today. There is no way to determine if it means a still birth or simply a premature birth. However, based on a comparison with Old Testament references to such a birth, we can infer that it likely does usually address a situation where the unborn does not survive.
Consider, for example, Psalm 58:8.
As a snail which melteth, let every one of them pass away: like the
untimely birth of a woman, that they may not see the sun. (Psalm 58:8,
This passage clearly refers to the "untimely birth" as addressing an individual who would "not see the sun." In other words, such a one does not survive.
Nor would the "untimely birth" be expected to survive in the cases depicted in Job or Ecclesiastes.
Or as an hidden untimely birth I had not been; as infants which never
saw light. (Job 3:16, KJV)
If a man beget an hundred children, and live many years, so that the
days of his years be many, and his soul be not filled with good, and
also that he have no burial; I say, that an untimely birth is better
than he. (Ecclesiastes 6:3, KJV)
Clearly, Paul did not die at birth, so this sort of "untimely birth" cannot be what he means.
And last of all he was seen of me also, as of one born out of due
time. (1 Corinthians 15:8, KJV)
The expression "as of one born out of due time" starts with the adverb "as". In Greek this is the word "ὡσπερεὶ," and it can mean "just as if" or "as it were." In other words, Paul is not intending to speak literally. As such, it cannot be assumed that he is speaking of an abortion which ends in the death of the unborn. It may well be that he is referencing a spiritual birth which has come well out of its time (whether early or late).
Based on the context, it seems he was actually coming late to a knowledge of Christ. Rather than being a preemie, Paul's (spiritual) birth was delayed.