Ramban on Leviticus 12:7

והקריבו לפני ה' וכפר עליה וטהרה ממקור דמיה יאמר שתקריב כופר נפשה לפני ה' שתטהר ממקור דמיה כי האשה בלדתה תהיה לה מעין נרפס ומקור משחת ואחרי עמדה בימי הנקיון או בימי יצירת הולד לזכר או לנקבה תביא כופר נפשה שיעמוד מקורה ושתטהר כי השם יתעלה רופא כל בשר ומפליא לעשות ורבותינו אמרו (נדה לא) בשעה שכורעת לילד קופצת ונשבעת לא אזקק עוד לבעלי ועיקר הכונה בזה כי בעבור שהיא נשבעת מתוך הצער ואין השבועה ראויה להתקיים מפני היותה משועבדת לבעלה רצתה התורה לכפר לה מעלות רוחה ומחשבות השם יתברך עמוקות ורחמיו מרובים שהוא רוצה להצדיק בריותיו: AND HE SHALL OFFER IT BEFORE THE ETERNAL, AND MAKE ATONEMENT FOR HER; AND SHE SHALL BE CLEANSED FROM THE FOUNTAIN OF HER BLOOD. Scripture is stating that she shall offer a ransom for her soul⁴⁰See Exodus 30:12. before the Eternal so that she shall be cleansed from the fountain of her blood, for a woman in childbirth has a troubled fountain and a tainted spring,⁴¹Proverbs 25:26. and after she has completed the number of days of becoming clean [as explained above in Verse 4], or the days of the formation of the child, male or female,⁴²This is a reference to Ibn Ezra’s explanation mentioned by Ramban (above, Verse 4). In other words, after she has completed the forty-day period for a male child and eighty days for a female — whether the reason that the Torah decreed these days is as Ramban explained it, in accordance with the opinion of the Sages, or as Ibn Ezra explained it in accordance with that of Rabbi Yishmael (ibid.) — she shall then bring etc. she shall then bring a ransom for her soul so that her fountain should be stayed, and that she should become cleansed, for G-d, praised be He, “heals all flesh and does wondrously.”⁴³Berachoth 60 b.
Now our Rabbis have said⁴⁴Niddah 31 b. that [the reason for these offerings is] that at the moment that she bends down to give birth she rashly swears [because of the pains of childbirth]: “I will no longer have relationships with my husband” [so as not to conceive again]. The main purport of this statement of the Rabbis is that since she only swears on account of her pain, and the oath is moreover not capable of fulfillment, because she is subject to her husband, therefore the Torah wished for her to atone for that which came into her mind [and therefore commanded her to bring these offerings]. G-d’s thoughts, blessed be He, are deep,⁴⁵See Psalms 92:6. and His mercies are bountiful, for it is His desire to justify His creatures.