Why did the midwife tie a red thread on the hand of the fetus that stuck out his hand first?
Seforno (to 38:27): It was to identify which of the twins came out first. 1 Moshav Zekenim, citing R. Yehudah ha'Chasid - Even though in any case, he is not [Yehudah's] Bechor, the custom was that the Bechor of a second wife received a field designated in her Kesuvah, for an extra portion. Also, it is relevant to redeem him from a Kohen for five Shekalim, which depends on Peter Rechem. Twins are prone to get mixed up. 2
Ohr ha'Chayim: She had Ru'ach ha'Kodesh and tied it, without knowing why. She [assumed and] said that he will come out first; the thread proved that it was not so.
Perhaps she suspected that it was a Divine Siman that he stuck out his hand, just like Yaakov came out holding Esav's heel (refer to 25:26:1:2 et. al). The Bechor was called Peretz because he overtook the one who stuck out his hand (PF)!
It was already known that she would have twins; Refer to 38:25:4:1, 38:25:4.1:1; 38:27:151:1. (She assumed that the one who stuck out his hand, had to come out first. Perhaps she tied it now, for after birth she will be busy cutting the umbilical cord.... - PF)
Peter Rechem depends on birth, i.e. when the head or majority leaves. It would have sufficed to make a Siman on one baby after full birth! (PF)
What is the Halachic significance of the fact that it was the midwife who declared, "Zeh Yatza Rishonah!"?
Yerushalmi Bava Basra, 3:1: It teaches us that the midwife is believed to say which twin is the Bechor. 1
See Torah Temimah, note 36, who elaborates.
QUESTIONS ON RASHI
Rashi writes: "'Va'Yiten Yad' - One of them put out his hand outside." What is Rashi explaining?
Gur Aryeh: An unborn child cannot "extend his hand" on purpose (in the same sense that an adult would). It simply means that his hand emerged from the womb.
Rashi writes: "'Va'Yiten Yad' - ... After she tied the red string on his hand, he brought it back." What is Rashi explaining?
Gur Aryeh: Why does the verse say, 'he put [out],' rather than using the term 'born'? It must be that this was not yet true birth, for he then pulled back his hand.
