On yesterday’s daf, we learned the case in the Mishna where two men offer kiddushin to two women, but at the time of the nesu’im (marriage) the women are switched and each ends up sleeping with the wrong spouse. Aside from the korban hatat (sin-offering) that they are obligated to bring, the families are obligated to wait a three month period in order to ascertain whose child the women are pregnant with, should it turn out that one or both of them conceive.
Our Gemara questions why this period of three months (havana) would be considered necessary, since it is known that a woman will not become pregnant the first time she engages in sexual relations. In answer, Rav Nahman suggests that the couples engaged in more than one act of relations and that the suspicion is that conception took place in a later act of relations.
Rava questions the very basis of the question, pointing out that there are known cases of virgins who had become pregnant the first time they engaged in sexual relations. Rav Nahman responded with the explanation that these women prepared themselves in advance by removing their hymen, which allowed them to become pregnant the first time they engaged in relations.
The arguments of the Gemara notwithstanding, modern medical opinion is that there is no physical impediment that would keep a woman who is a virgin from becoming pregnant the first time she has sex. Nevertheless, it is not unreasonable to assume that there are both physical and psychological barriers that lessen the likelihood of a woman becoming pregnant under such circumstances. The testimony of the Gemara about certain women who “prepared themselves” to become pregnant the first time they had sexual relations likely means that they not only physically prepared themselves, but that they also prepared themselves emotionally for the intimacy of the sexual act.