In "Word Focus" Darrell Grim writes:
Paul wrote that “the day of the Lord so cometh as a thief in the night” (1 Thess. 5:2). John Gill writes, “Respect is had not to the character of the thief, nor to the end of his coming; but to the manner of it, in the dark, indiscernibly, suddenly, and when not thought of and looked for; and such will be the coming of Christ, it will be sudden, and unknown beforehand, and when least thought of and expected.” In 1 Thessalonians 5:3, Paul used the analogy of a pregnant woman whose labor pains come upon her suddenly. It seems that he was indicating the coming of the Lord will be sudden and quite painful to those who are not ready to meet Him. A pregnant woman may have a general idea when she is due to give birth, but the exact time can be a surprise. Paul's message to the Thessalonians was the same as Christ's message to John in The Revelation. He will come back suddenly and without warning, especially to those who are not prepared and watching for Him.
Source: Christ, the Triumphant Lord: Adult Teacher's Insights, page 75.