Is the Halacha different regarding Onos that are within a couple of months post birth? Must they all be kept? Is the difference only regarding Veses Kavua. How does it work?
After a birth one must keep the Onos (Hachodesh, Beinoni, Hafloga) only after she started having her period again (which usually will not happening for a couple of months after a birth).
Those Onos are to be kept as an “unfixed cycle”.
One can not set up a fixed cycle during 24 months after a birth, even if 3 periods happen at the same date and Onah.
Y”D 189 33