Yevamot 80B

מפני הסכנה. הכא במאי עסקינן ⁠— כשגמרו סימניו. דתניא: איזהו בן שמנה? כל שלא כלו לו חדשיו. רבי אומר: סימנין מוכיחין עליו, שערו וצפרניו שלא גמרו. טעמא דלא גמרו. הא גמרו ⁠— אמרינן האי בר שבעה הוא, ואישתהויי הוא דאישתהי.

due to the danger, both to the baby, who might in fact be viable, as well as to the mother, who might suffer fatal complications if she has to retain all her milk. The Talmud answers: With what are we dealing here? We are dealing with a case where his signs of viability are fully developed, and he has the appearance of a viable child. As it is taught in a baraita: Who is a baby born during the 8th month? It is anyone whose months of gestation have not been completed, i.e., a baby that was born prematurely. R' Yehuda HaNasi says: The signs that prove that the child falls into this category are that his hair and nails are not fully developed. Now, the reason is that they are not fully developed; but if his hair and nails are fully developed, we say that this fetus was fit to be born after 7 months but for some reason was delayed in its mother’s womb.

אלא הא דעבד רבא תוספאה עובדא באשה שהלך בעלה למדינת הים ואישתהי עד תריסר ירחי שתא, ואכשריה. כמאן ⁠— כרבי, דאמר משתהא!

But if so, with regard to the action taken by Rava Tosfa’a concerning a woman whose husband went overseas and her baby was delayed in her womb for the 12 months of the year following her husband’s departure, and Rava Tosfa’a rendered the child fit, arguing that the husband is presumed to be the father and the child is not a mamzer, according to whose opinion did he issue this ruling? It must have been in accordance with the opinion of R' Yehuda HaNasi, who says that a baby can be delayed for an extended period of time in its mother’s womb even after it is fully developed and ready to be born. But how could Rava Tosfa’a have ruled in accordance with the minority opinion of R' Yehuda HaNasi, against the majority opinion of his colleagues?