Sforno on Genesis 38:18/ספורנו על בראשית ל״ח:י״ח

1ופתילך, your closely fitting sash. The items Tamar chose as pledge were all things which testified to the superior standing of its owner in society. We know from Job 38,3 that a sash is evidence of someone’s manhood, i.e. someone’s superior status. She wanted to own such trinkets reminding her constantly of Yehudah’s status so that her child would be influenced by the thoughts she entertained during her pregnancy and would grow up to be like its father.