The fortieth day is a part of child birth. Accordingly, this divorce is Bid`i. Your husband must revoke divorce and issue it again when you become pure, i.e. after the expiry of your menses. If you are not sure whether you were pure or not when he issued divorce, you must refer to the default rule, i.e. you were in a state of impurity which was certain. You cannot cancel the state of certainty by means of probability or conjecture.

Allah Almighty knows best.