The definite sign of placental separation in stage 3 of paurition is

Placental separationMarked retraction reduces effectively the surface area at the placental site to about its half. But as the placenta is inelastic it cant keep pace with such an extent of diminution resulting in its buckling.The detachment of the placenta from the uterine attachment stas in the center resulting in opening up of few uterine sinuses and accumulation of blood behind the placenta.Marginal separation occursD.C.DUTTA&;S TEXTBOOK OF GYNECOLOGY, Pg no:122,7th edition