Why is the waiting period for a slave woman half that of a free woman?

Question:
If the waiting period is intended to ensure the purity of the womb, how can the waiting period for a slave woman be half that of a free woman? Does her womb become pure before the free woman’s, for example?

Answer:
Purity is achieved with one menstruation. So Allah made the waiting period for a free woman three menstruations as a precaution for the religion and for the purity of the womb, and to protect the woman after divorce from any suspicion that might attach to her afterward. Thus, Allah made three menstruations the limit in caring for the purity of the womb and as a precaution for marriage, so that she may marry afterward with certainty and clear understanding. As for the slave woman, since her status is lower, her waiting period was made half. Since the second menstruation was not half, it was set at two menstruations. There is no harm in this, and the Companions of the Prophet (peace and blessings be upon him) followed this practice, and they were more knowledgeable than us and had a clearer understanding of the religion.