The term placental sign denotes :

All the pregnant females have amenorrhea . In a few pregnant females however, cyclic bleeding occurs upto 12 weeks of pregnancy i.e,until the decidual space is obliterated by the fusion of decidua vera with decidua capsularis. Such bleeding is usually scanty, lasting for shoer duration than the usual cycle and roughly corresponds with the date of the expected period. This is called placental sign. Ref:Datta Obs 9e pg 60.