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Our Gemara questions why a period of three months (havchanah) would be considered necessary before remarriage, since it is known that a woman will not become pregnant the first time she engages in sexual relations.
Rav Nachman 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 example of Tamar is brought to disprove the notion of the impossibility of pregnancy after a single act of cohabitation, which allows us to explore further the concepts of conception in antiquity.
The Noda B’Yehudah writes in his responsa נודע יהודה קמא אה"ע סימן כ"ב that nowadays it is possible for women to become pregnant the first time they have relations.
Furthermore, even in the time of Chazal the principle did not indicate that it was impossible for a woman to become pregnant the first time she had relations, rather the principle was stated with regards to the majority.