Why did Yehudah call the first baby 'Peretz'?
Rashi, Targum Onkelos and Targum Yonasan: Because he applied great force to emerge first from the womb.
Ramban #1: Basically, Peretz means to breach a fence. 1 However, in a wider sense, it is used with regard to someone who extends himself, 2 such as here, where Peretz rushed to overtake his brother, who was initially ahead of him. 3
Ramban #2 (citing a Midrash): Esoterically speaking, Zerach hints at the sun, and Peretz, at the moon (the symbol of Malchus Beis David). 4
Ohr ha'Chayim: The first one (Zerach) stuck out his hand, for he was conceived last (and should have been born first - refer to 25:26:1:1). Peretz 'breached' nature, and came out first, even though he was conceived first, since kingship was designated for him.
Malbim (to 38:27): Peretz and Zerach embodied Er and Onan, respectively, through the secret of Yibum. Zerach wanted to leave first, because Onan was not as evil as Er. Peretz breached through and came out first, since Er was the Bechor, and Malchus Beis David therefore descends from him.
Ha'amek Davar: Peretz breached to overtake the first, and also endangered himself. This hints to a king, who breaches a path for himself, and endangers himself in war.
Like we find in Tehilim 80:13. See also Sukah 26a.
Ramban: See for example, Bereishis 28:14, 30:43 .
Ramban: The midwife added the word "Alecha," as if to say that he broke through the wall that held him back.
Which explains why we say during Kidush Levanah, 'David Melech Yisrael Chai v'Kayam!' (See Ramban). Also see Gur Aryeh (refer to 38:25:4.1:1 and the notes there).
