Or HaChaim on Genesis 38:29/אור החיים על בראשית ל״ח:כ״ט

1 מה פרצת עליך פרץ, "how did you burst your way out?" Inasmuch as the hand of the first one indicated that he was about to emerge from the womb before his brother why did you push your way out past him thus blocking his becoming the first-born? The word מה, which normally means "what," must be understood here similarly to Psalms 104,24: מה רבו מעשיך, "how great are Your works, etc.?" When the Torah continues עליך פרץ, this explains the nature of this bursting forth. Normally, when twins are born, the one which emerges first is from the sperm that entered the womb last, the earlier sperm being blocked by the sperm injected later. In this instance the baby born first was formed from the sperm that had entered Tamar's womb first. This is why the kingdom rightfully belonged to Peretz, seeing he had been both conceived first and born first.