Sforno on Genesis 30:8/ספורנו על בראשית ל׳:ח׳

1נפתלי אלוהים נפתלתי, the word נפתולי is a variant of the word פתיל as in צמיד פתיל, a tightly fitted lid (Numbers 19,15) Rachel meant that her cleaving to her husband was an attachment containing spiritual, religious dimensions, making the bond between them immeasurably stronger.

2עם אחותי, I did this jointly with my sister, both of us having liberated our respective handmaids in order to enable them to become mothers of the founding fathers of the Jewish people. This is what they meant when considering the sons of these former handmaids as בנים instead of as עבדים. Accordingly, Leah had already liberated her handmaid Zilpah before Bilhah had given birth to Naftali. How else could 8 of the 12 tribes have been born within 7 years of Yaakov’s getting married? 6 of these sons were Leah’s and 2 were Zilpah’s. Assuming each pregnancy lasted 9 months, and further assuming that none of these pregnancies occurred simultaneously, 8 times 9 months amounts to 72 months, i.e. 6 years. Unless Leah had given her handmaid to Yaakov before Naftali was born there would not have been enough time for Joseph to have been born at the end of the second 7 years of Yaakov’s service. At that time, after Joseph’s birth, Yaakov requested permission to go home to his parents.(3,26). The only alternative to our calculation would be that all the children were born after only 7 months pregnancy, and that none of the mothers observed even basic days of ritual impurity before again sleeping with their husbands.

3גם יכולתי, I have achieved what I had planned by giving my maidservant to Yaakov.