I am a student. I think pregnancy has nothing to do with divorce but with the next marriage. Before next marriage, it is important to make sure that the woman is not pregnant.
To my knowledge, the waiting period for divorce finalization is to allow cooling down of emotion in case a couple wants to change their decision and maintain the marriage before a divorce is final
Having periods is no guarantee that a woman is not pregnant. There are thousands of documented cases where woman can be pregnant and still have periods. Even if with a good scientific test it could be determined with certainty that a woman is not pregnant, still the couple is required to complete the waiting period before a divorce is finalized; unless it is the third time a couple is going through divorce. In that case there is no waiting period before a divorce is finalized, but a waiting period for a woman before getting married with another person.