רמב"ן — תביא כבש בן שנתו / וכפר עליה וטהרה ממקר דמיה

Q — Why does the yoledet bring a korban — what sin did she commit? Chazal: in labor she swears she won't return to her husband — and the korban atones for that. But the oath isn't valid (she's bound to him), and the korban doesn't fit the standard shevua-pattern (which would be different categories of animal).

A — Ramban offers a striking pshat: the yoledet's korban isn't a chatat-style atonement at all. It is 'כופר נפש' — a propitiatory offering accompanying a prayer for healing: 'שיעמוד מקורה ושתטהר.' Childbirth has rendered her with 'מעין נרפס ומקור משחת'; the korban is offered with the hope that Hashem 'רופא כל בשר ומפליא לעשות' will heal her. The phrase 'וכפר עליה וטהרה ממקור דמיה' isn't sin-atonement-yielding-purity, but a tefilla that the offering should issue in cessation of bleeding and restoration to taharah. (As a secondary reading, Ramban also explains Chazal's vow-account: not literal kapara for the vow, but for the 'מעלות רוחה' — the troubled thoughts produced by labor's distress.)