Concerning Divorced Women 2:228

And the divorced women possessing menses, it is necessary that they wait before marrying after pronouncing divorce; a period of three purifications or three menses in accordance with the way of the waiting period; in order to confirm the emptiness of the womb from pregnancy. And it is not permitted for them to marry another man during the course of this waiting period until it is concluded.

And It is not permissible for them that they keep secret; what Allah created without a previous example before anything existed in their womb from pregnancy, or menses, if the divorce women are truly believers in Allah and the Day of Standing.

After the three months she can obtain a divorce if she is not pregnant. If she is pregnant with life in her womb, then the knowledge that she has new life in her should change the husband and the wife. Thus, they should not decide to get a divorce but should decide to maintain their marriage relationship.