Adam Clarke on Genesis 38:28

The midwife - bound upon his hand a scarlet thread - The binding of the scarlet thread about the wrist of the child whose arm appeared first in the birth, serves to show us how solicitously the privileges of the birthright were preserved. Had not this caution been taken by the midwife, Pharez would have had the right of primogeniture to the prejudice of his elder brother Zarah. And yet Pharez is usually reckoned in the genealogical tables before Zarah; and from him, not Zarah, does the line of our Lord proceed. See Mat 1:3. Probably the two brothers, as being twins, were conjoined in the privileges belonging to the birthright.