Although the Gemara related a series of rules of which Sage’s opinion to follow when there are disagreements (see 46a-b), Rav Mesharshiya rejects these rules out-of-hand. The Gemara lists a number of cases that may act as a source for Rav Mesharshiya’s position.
One of the cases involves a gezeira obligating a woman to a three-month waiting period between the end of one marriage and the beginning of another. The Mishna in Yevamot (41a) teaches that a yevama – a woman whose husband died, leaving her childless, and whose brother-in-law (her late husband’s brother) is obligated by the Torah to take her as his wife – should neither marry him nor undergo the halitza ceremony that will allow her to marry someone else, until three months have passed since her husband’s death. This rule applies to cases aside from Yevamot, including cases of divorce or widowhood where there are children from the first marriage.
The purpose of the Rabbinic injunction obligating a three-month wait between marriages is to ascertain beyond any doubt the parentage of any possible child born to the woman in question. We want to make sure that we know whether her first husband is the father of the child, or if it is the second one. The choice of three months stems from a number of different considerations. Based on the Biblical story (Bereishit 37) of Yehuda and Tamar, it is clear that, at three months, a pregnancy can become visible. Also, based on the Gemara’s tradition that there are two types of pregnancy – a seven month pregnancy and a nine-month pregnancy (see 129-135). According to that assumption, were the woman permitted to remarry immediately, we would not know whether the newborn is a nine-month baby from the first marriage or a seven-month baby from the second marriage. The three-month waiting period clarifies who is the true father.