The uterus becomes pelvic organ after delivery in:

Uterus begins to shrink in size 2 days following delivery and it becomes a pelvic organ after 2 weeks. By 6 weeks uterus regains its nonpregnant size. During involution of uterus there is no loss of myometrial cells. It occurs due to diminution of size of myometrial cells as a result of enzymatic autolysis of cytoplasm. There is augmentation of protease activity. Each myometrial cell measuring about 500-800ml in late pregnancy shrinks to less than 100ml. Ref: Donald School Manual of Practical Problems in Obstetrics edited by Narendra Malhotra page 581.