What are the exceptional disease/diseases, requiring post partum hysterectomy?

If you're talking postpartum as in IMMEDIATE postpartum, that would be due to uncontrolled hemorrhage, which could be happening in an intact uterus, or in a uterus that has ruptured during labor / birth. All attempts would first be made to stop the bleeding, but sometimes it just cannot be done, and hysterectomy is indicated.

In the later postpartum period, severe infection could be a cause. This would also be extremely rare and all measures to treat the infection would first be undertaken. Usually bleeding/hemorrhage is a co-existing problem at this time as well.

But there are many other reasons why hysterectomy might be indicated in women who are not in the postpartum state, but I won't list them since you specifically asked about postpartum hysts.

For me, I had PCOS, which caused severe period pains, and really heavy periods and they were irregular, so I went to my doctor and got checked out and consulted with my doctor and we agreed that having a hysterectomy would be the best, and I am so glad I had it to this day.♥♥♥