The term "placental sign" denotes :

Cyclic bleeding may occur upto 12 weeks of pregnancy, until the decidual space is obliterated by the fusion of decidua vera with decidua capsularis. Such bleeding is usually scanty, lasting for a shoer duration than her usual and roughly corresponds with the date of the expected period. This is termed as placental sign. This type of bleeding should not be confused with the commonly met pathological bleeding, i.e. threatened aboion. Reference: Dutta's Textbook of obstetrics 7th edition page no 71