John Gill Commentary Genesis 38:28

"And it came to pass, when she travailed, that one put out a hand: and the midwife took and bound upon his hand a scarlet thread, saying, This came out first." — Genesis 38:28 (ASV)

And it came to pass when she was in labor Her birth throes came strong and quick upon her: that [the one] put out [his] hand; which showed that she was likely to have a difficult and dangerous time of it; that the birth was not likely to be according to the usual and natural order, which may be considered as a correction for her sin: and the midwife took and bound upon his hand a scarlet thread, saying, this came out first; she tied this to his wrist, that she might know whose hand it was, and so which was the firstborn; which, to know was a matter of consequence, since to the firstborn there were some special and peculiar privileges.