Genesis 38:28
And as she was giving birth, one of them put out his hand; so the midwife took a scarlet thread and tied it around his wrist. “This one came out first,” she announced.
What it says
As she gives birth, one child puts out his hand, and the midwife ties a scarlet thread around it to mark him as first.
What it is doing
It establishes the expected order of birth.
In context
27When the time came for Tamar to give birth, there were twins in her womb. 28And as she was giving birth, one of them put out his hand; so the midwife took a scarlet thread and tied it around his wrist. “This one came out first,” she announced. 29But when he pulled his hand back and his brother came out, she said, “You have broken out first!” So he was named Perez. 30Then his brother came out with the scarlet thread around his wrist, and he was named Zerah.