They’ll be part of that false "Peace and safety" because they will be fooled into thinking Jesus Christ has already returned, when it will be a false messiah. Thus "they shall not escape". They'll be 'taken' alright, but not by Christ Jesus, the True Bridegroom. That's why they will be 'with child' spiritually speaking, even thinking that they've 'escaped' the great tribulation, but they will not escape. When our True Lord and Savior Jesus Christ does return afterwards, those will be 'with child' spiritually, because they will have already married a bridegroom, but a false one, i.e. false messiah (Lucifer playing the role of the 'another Jesus'. see 2 Cor.11).

1Thes 5:4    But ye, brethren, are not in darkness, that that day should overtake you as a thief.
5    Ye are all the children of light, and the children of the day: we are not of the night, nor of darkness.
6    Therefore let us not sleep, as do others; but let us watch and be sober

 

     This 'darkness' is that same 'night' symbol of Isa.21:4 which brought us here. It is a spiritual 'darkness', one of spiritual sleep. Paul is telling his brethren in Christ who know the "times and the seasons" of our Lord's coming, that they are not in a spiritual stupor (darkness). So Christ's second coming will definitely NOT be "as a thief in the night" to God's elect. Moreover, Paul is here telling God's People to be watching, and be sober, awake, watching for the signs our Lord gave us.
     So the birth pangs Message within God's Word goes much deeper, and I wanted to make sure it was well covered. That many from all nations will experience the 'pangs', and be spiritually 'with child' when our True Lord Christ does return, lets us know that no lone nation of people can claim to be in God's perfect 'grace' during the future tribulation. Our Heavenly Father is going to show us in the near future just how far we can, and have drifted away from Him and His Truth. And this Message joins with these 'burden' Chapters we've been covering here in Isaiah; burdens which all nations bring upon themselves due to their own flesh thinking, allowing themselves to be mystified by Satan's own false workers, and thus becoming 'apostate' away from God. We are all God's children in this sense, so we need to seek only ONE during this time. That ONE is Christ Jesus, our Savior.
     Now back to Isaiah 21:5:

Isa 21:5   Prepare the table, watch in the watchtower, eat, drink: arise, ye princes, and anoint the shield.
6    For thus hath the Lord said unto me, "Go, set a watchman, let him declare what he seeth."

     This is given in warning from God to the false princes who were to be righteous leaders to His People. But they set up their own gods, their own laws, their own moral code, etc. God's saying something like, "Go ahead you princes of the people, set yourself a great big old' table feast. Enjoy it while you can, for it's all coming down very soon. Anoint your 'own' shield of Baal worship, instead of My Shield of Faith!"
     Then God tells them to go setup a 'watchman' in a tower, a lookout.

Isa 21:7   And he saw a chariot with a couple of horsemen, a chariot of asses, and a chariot of camels; and he hearkened diligently with much heed:

     This word 'chariot' is Hebrew 'rekeb', and means 'a vehicle; by implication, a team, by extension, cavalry' (Strong's no. 7393). These are the Persians and Medes, who were known to haul much of their hardware with them for war upon camels and asses. In other words, they setup a very large base camp to carry out operations from and were prepared for a long battle.
     Recalling from Isaiah 13 about God's anointed one of this time, i.e. Cyrus, how may this apply to our near future? Cyrus is a 'type' for our Lord and Saviour’s Role as KING in the near future, coming to destroy the future Babylon false system spread over all the earth. Our Lord's 'sanctified ones' who make up His Army, having His Gospel Armour, are God's elect upon the earth today and the saints now in Heaven who return with Jesus. The final battle is a spiritual battle.