וַיִּמְלְא֥וּ יָמֶ֖יהָ לָלֶ֑דֶת וְהִנֵּ֥ה תוֹמִ֖ם בְּבִטְנָֽהּ׃
그 해산 기한이 찬즉 태에 쌍동이가 있었는데
Siftei Chakhamim
They completed the days of his embalming until forty days. Rashi is answering the question: [Why does it say וימלאו?] The term מלוי (completion) applies only to a predetermined number, such as: “I will fill (אמלא) the number of your days” (Shemos 23:26), meaning the number of days apportioned to you. Similarly (25:24): “When her days of pregnancy were completed (וימלאו),” meaning the number of days apportioned for her pregnancy. There are many [other examples of this] in Scripture. But here, there was no predetermined number, so Rashi needed to add: “They completed the days of his embalming.” Their period for embalming was for a predetermined number of forty days, as Rashi goes on to say. Rashi says השלימו instead of וימלאו, to teach that this מילוי denotes “completion,” rather than denoting “full” which is the opposite of empty. (Re’m)
Tze'enah Ure'enah
That is why it is written, “there were twins” [25:24], without an aleph, because one was an evildoer and the other one was righteous. However, concerning Tamar, they were both righteous, Peretz and Zerach. Therefore, it is written concerning Tamar, twins, with an aleph.18Bahya, Genesis, 25:24.