“Now when they are saying, ‘There is peace and security,’ then sudden destruction comes on them, like labour pains on a pregnant woman, and they will surely not escape. But you, brothers and sisters, are not in the darkness for the day to overtake you like a thief would.” 1 Thess 5:3–4
Paul’s reassurance to the Thessalonians implies that they were anxious that they might not escape the day of the Lord and therefore endure God’s wrath. Paul confirms Christians will be raptured before God’s wrath is poured out on an unbelieving world. It is at the end of the sixth seal being opened that His wrath commences: “For the great day of His wrath has come who can stand.” (Rev. 6:17) The seven trumpet and seven bowl judgements are the wrath of God.
Note, Paul contrasts the ungodly, who will not escape because Christ is coming back as a thief for them (as in the days of Noah) , with believers who are obedient-vigilant and “not in the darkness for the day to overtake you like a thief would.” Incidentally, this reassuring promise undermines the pretribulational notion that Jesus is secretly coming back as a thief for his church.
During the Antichrist’s great tribulation, the world will experience peace and security for those who are loyal to him. Hence, Paul prophesies that the world will be saying, “There is peace and security” before Jesus returns to mete out His day of the Lord’s wrath upon the wicked.
The church will be here to witness the ungodly exclaiming that they are experiencing peace and safety. This prophecy that Paul mentions undermines imminence because it will happen before, not after, the rapture.
This is just another prophesied event in a long series of intervening events that will happen before the rapture—rendering the pretribulation teaching of imminence contradictory in light of this biblical evidence. 
The peace and safety will be illusory, a false security for unbelievers, because eventually unforeseen calamity will come upon them just as unexpected labour pains come upon a pregnant woman.
We should be careful not to confuse Paul’s use of the birth pangs analogy with Jesus’ purpose in using the same phrasing in the Olivet Discourse (“All these things are the beginning of birth pains,” Matt 24:8). Paul uses the phrase in a completely different application. Jesus applies the birth pangs metaphor to particular events before the .Antichrist’s great tribulation, while Paul applies it to the situation of the onset of the day of the Lord’s wrath after the great tribulation.
For a more in depth coverage go to alankurschner.com – The Prophecy – “There is peace and security” contradicts Pretribulation Theology.