We know exactly what Paul meant by the word because he immediately explains it:
For I am the least of the apostles, unworthy to be called an apostle, because I persecuted the church of God.—1 Corinthians 15:9 (ESV)
Therefore Paul means us to understand that when Jesus appeared to him on the road to Damascus (see Acts 9), he was not worthy to be called an apostle. Thus he is not claiming to be the victim of an abortion in any literal sense; he must be speaking metaphorically. Of course, the phrase in verse 8 makes clear via the word "as" that a simile is intended. So it would probably be best if the non-metaphorical meaning were used to bring out Paul's full meaning.
An "untimely birth" sounds like Paul was "born under a bad sign" or doomed by fate. Those make some sense as analogies to one who is unworthy. But the context of the entire chapter is resurrection: rebirth. Paul's former life was a dead end, so to speak, and so it makes even more sense to call himself "stillborn" or even "an abortion". When Paul talks about his former life, he talks about being spiritually dead.
Paul actually talks about himself being fortunate at birth from a post-conversion vantage point:
But when he who had set me apart before I was born*, and who called me by his grace, was pleased to reveal his Son to me, in order that I might preach him among the Gentiles, I did not immediately consult with anyone.—Galatians 1:15-16 (ESV)
The ESV note is:
Greek set me apart from my mother's womb
The maximal contrast between Paul's old life and his new one, comes when you compare his self-designation as an aborted fetus to the resurrection body:
For this perishable body must put on the imperishable, and this mortal body must put on immortality. When the perishable puts on the imperishable, and the mortal puts on immortality, then shall come to pass the saying that is written:
“Death is swallowed up in victory.”
“O death, where is your victory?
O death, where is your sting?”
—1 Corinthians 15:53-55 (ESV)
According to Barnes' Notes on the Bible, the Greek word is used in the Septuagint to translate "stillborn" (nephel <05309>) in Job 3:16 (NJPS):
Or why was I not like a buried stillbirth,
Like babies who never saw the light?
and Ecclesiastes 6:3 (NJPS)
Even if a man should beget a hundred children and live many years—no matter how many the days of his years may come to, if his gullet is not sated through his wealth, I say: The stillbirth, though it was not even accorded a burial, is more fortunate than he.
Both of these use strong imagery to highlight the unworthiness and misfortune of the subject.
Translators should not shy away from vivid, even disturbing, images when rendering the Bible to a target language. The word ektroma should be translated "an abortion" in 1st Corinthians 15:8.