Uterine linings are sign of fertility in females. Endometrium has a 1 mm thick layer. This layer starts to proliferate after periods under the influence of a hormone called estrogen. After the ovulation at the mid of the cycle, this proliferated endometrium is transformed under the influence of another hormone called progesterone. This progesterone is necessary to maintain the linings of uterus. When progesterone levels drop below bleeding threshold, these uterine linings start to shed in the form of periods. That is why if a woman takes oral progesterone starting from 3 days before the expected date of periods then uterine linings are maintained and periods are postponed until tablet taking continues.