Delivery by cesarean section is not called nifās

Answer: One can argue on their behalf that what is customary in the Sharia is that the rulings of nifās are predicated upon two successive matters:
1. the exit of the child from the specified orifice; and
2. the exit of blood immediately after it.
There is no doubt that these two together are the cause and sign for the rulings of nifās. Neither the exit of blood alone, nor the exit of the child alone, is a cause; rather, the combination is the cause.
The proof is that blood exiting from the vaginal orifice in the period before delivery does not entail any of the rulings of ḥayḍ or nifās—for it has been related that menses does not co-exist with pregnancy, as it was made provision for the fetus (or as is narrated from the Commander of the Faithful).
And the exit of the child alone is not called nifās nor does it entail the rulings of nifās.
Moreover, the textual evidence concerns blood exiting from the specified orifice immediately after the child exits from that same orifice. Therefore, when the child exits by a cesarean operation, and then blood exits from the vagina afterward, this would require evidence establishing that it carries the same ruling as the first case; no such evidence exists, and analogy (qiyās) in such matters does not avail. Do you not see that it is invalid to analogize the blood of istiḥāḍah to menstrual blood, or blood before delivery to blood after delivery? Many scholars of legal theory have prohibited analogy with respect to legal causes (asbāb).
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1