What is meant by "after she has purified her womb," and is it permissible to renew the contract without purifying the womb?

Istibra’ al-rahm (ensuring the womb is clear) means that a woman menstruates and then becomes pure. This is achieved, according to the more preponderant opinion, with a single menstrual period. The marriage contract is not valid before this according to the view that holds it obligatory. However, if the contract is concluded before her istibra’, it proceeds out of consideration for those who deem it valid, but cohabitation is not permissible until her womb has been cleared.