Our Gemara asks: From when do we begin counting for a divorce? Rav says the significant moment for the geṭ is when the geṭ is handed over; Shmuel says that it is from the time that the geṭ is written. After some discussion the Gemara concludes that we can begin counting from the time that it is written.
The need to know when we can begin counting “for the geṭ” refers to a period called havhana (literally, “a period of distinction”) – three months from the time that the couple has separated.
This rule appears in the Mishna in Massekhet Yevamot (41a), which teaches that a woman whose husband passes away with no children will receive neither yibum nor halitza for three months after his death. Furthermore, according to the Mishna, even in non-yibum situations this rule applies, whether the woman was divorced or widowed, whether the first marriage ended after erusin or nissu’in.
These three months are essential in order to clarify who is the true father. Were a woman who was already pregnant at the time that her first marriage ended to get married immediately, we might mistakenly think that the child’s father is the second husband, a mistake that can lead to problems regarding forbidden relations (for example, not knowing who the child’s true siblings are) as well as mistakes in inheritance law. Moreover, the very purity of the Jewish family will be threatened by the fact that children will be unsure about who their father is. By waiting three months, even if the woman is pregnant it will be obvious, and if she gets married it will be clear that the child is the progeny of the first husband and is not a child from the second husband born a month or two early.