5:1-3
1-2: Now concerning the times and the seasons, brothers, you have no need to have anything written to you. For you yourselves are fully aware that the day of the Lord will come like a thief in the night.
Paul is continuing his train of thought from the end of chapter four. He is reminding the Thessalonians about what they already know concerning the end times.
3: While people are saying, “There is peace and security,” then sudden destruction will come upon them as labor pains come upon a pregnant woman and they will not escape.
I don’t think I fully understood this verse until I experienced labor pains myself. One minute you’re fine and dandy and the next minute you have searing pain rippling across your back and stomach. I’ve read multiple passages where the end times are compared to labor pains. This is because they come on suddenly and progress to be closer and closer together until the child is born. Just as a woman cannot predict when labor will start, we cannot predict when destruction (and the second coming of Christ) will arrive.