Reply to Objection 2: Moses seems to have chosen his words in order to exclude uncleanness from the Mother of God, who was with child "without receiving seed." It is therefore clear that she was not bound to fulfil that precept, but fulfilled the observance of purification of her own accord, as stated above.
Reply to Objection 3: The sacraments of the Law did not cleanse from the uncleanness of sin which is accomplished by grace, but they foreshadowed this purification: for they cleansed by a kind of carnal purification, from the uncleanness of a certain irregularity, as stated in the FS, Q[102], A[5]; FS, Q[103], A[2]. But the Blessed Virgin contracted neither uncleanness, and consequently did not need to be purified.