Rabbah bar Sheila quotes Rav Matanah in the name of Shmuel as teaching that they once identified a placenta as connected to a birth that had taken place ten days before. Rabbah bar bar Hanna quotes Rabbi Yohanan as teaching that they once identified a placenta as connected to a birth that had taken place 23 days before (Rav Yosef argued that it was 24 days). Rav Aha brei d’Rav Avira quotes Rabbi Yitzhak as teaching that once the birth of a second child followed the first birth that had taken place 33 days before (Rav Yosef argues that it was 34 days).
Finally, Rabbi Avin bar Rav Ada quotes Rav Menahem ish Kfar She’arim (or, perhaps, Bet She’arim) as saying that once the birth of one twin was delayed three months after his older brother was born. Moreover, they were sitting in the Bet Midrash at that moment – Yehudah and Hizkiyah the sons of Rabbi Hiyya.
These statements, and, in particular, the testimony regarding Yehudah and Hizkiyah the sons of Rabbi Hiyya, appear to present a problem to Rav who ruled that two children in a single pregnancy are always born immediately after one another (see yesterday’s daf). This is particularly difficult, since Rav was Rabbi Hiyya’s nephew and must have known the circumstances of his cousins’ birth.
Tosafot explain that Rav recognized the possibility that there may be a longer time difference between births, but since it was such an unusual occurrence it was not taken into consideration by the halakha.
The Ran suggests that Rav’s statement only applied to situations where both fetuses were fully developed. If one had not yet developed, it may remain in utero even for an extended period of time.
In the Gemara, Abayye explains the circumstances of Yehudah and Hizkiyah’s birth based on the Rabbinic idea that normal gestation cycles were either seven or nine months long. Although this approach is rejected by modern science, the possibility of superfetation – of double pregnancy where a fetus is formed during an existing pregnancy – does exist in rare cases.