Bible pretty much interprets itself...
Mic 4:10 Be in pain, and labour to bring forth, O daughter of Zion, like a woman in travail: for now shalt thou go forth out of the city, and thou shalt dwell in the field, and thou shalt go even to Babylon; there shalt thou be delivered; there the LORD shall redeem thee from the hand of thine enemies.
Many Hebrew and Greeks words are translated as travail...In this context, travail refers to the child bearing process which is associated with the pain of childbirth...
1Th 5:3 For when they shall say, Peace and safety; then sudden destruction cometh upon them, as travail upon a woman with child; and they shall not escape.
Here, travail means pain...
ὠδίν
ōdin
o-deen'
Akin to G3601; a pang or throe, especially of childbirth: - pain, sorrow, travail.
Sometimes travail mean toil but not when used in the process of child birth...
Some would consider toil to be a pain. We are just talking about degrees of pain and each individuals level of tolerance here, all for the purposes of analogy..or getting people to understand the authors point from a common, human, gut-level understanding (recognition).
Pain is anything that causes the body stress or takes from it..childbirth is one level, one horrendous but fleeting level, yet not the worst ( I'm sure this is totally subjective and purely debatable).
"Travail" is simply a French word,travaille anglicized, and meaning "work".
It is an observation, no number of holy books to the contrary notwithstanding, that parts of the process or parturition are not painful.
By the figure of metonymy and by the usual coincidence of pain and parturition, one might say "travail" and mean "pain". In the quote from Thessalonians I would say the point is not the pain but the sudden, seemingly arbitrary, and utterly irresistible quality of childbirth.
I have watched ewes hold back on labor for as much as an hour. They will sit there giving me the hairy eyeball, and if I step away for a 5 minutes, when I come back there are twins on the ground!
But for human type personnel, the normal case seems to be that once it starts there's no stopping it. I think that's what Paul was talking about: sudden and unstoppable.
(Isn't "throe" a great word?)