What sign was given for the time of Jacob's trouble? A baby being born? The time of Jacob's trouble is when Herod had all the young infants killed, Rachel weeping for her children. And who was it that was saved out of that, that was carried away to the land Egypt while Herod yet lived?
Jeremiah 30:7
Alas! for that day is great, so that none is like it:
it is even the time of Jacob's trouble;
but he shall be saved out of it.
Jeremiah 31:15
Thus saith the LORD;
A voice was heard in Ramah,
lamentation, and bitter weeping;
Rachel weeping for her children
refused to be comforted for her children, because they were not.
Matthew 2:16-18
Then Herod, when he saw that he was mocked of the wise men, was exceeding wroth, and sent forth, and slew all the children that were in Bethlehem, and in all the coasts thereof, from two years old and under, according to the time which he had diligently enquired of the wise men. Then was fulfilled that which was spoken by Jeremiah the prophet, saying, In Rama was there a voice heard, lamentation, and weeping, and great mourning, Rachel weeping for her children, and would not be comforted, because they are not.
This was the time of Jacob's trouble, Rachel, his wife weeping for her children. There were no signs leading up to this, but it was a sign, a prophecy fulfilled.