Reference for Yevamot 73:8

א"ל הכי קאמינא רוב נשים ילדן לתשעה ומיעוט לשבעה וכל היולדת לתשעה עוברה ניכר לשליש ימיה וזו הואיל ולא הוכר עוברה לשליש ימיה איתרע ליה רובא

IF IT IS DOUBTFUL WHETHER IT IS A NINE-MONTHS CHILD etc. Said Raba to R. Nahman. Let the ruling be that one is to go by the majority of women, and the majority of women bear at nine months!<span class="x" onmousemove="('comment',' The child would consequently be deemed to be the son of the first husband, and the marriage of his mother with the levir would be a forbidden union. The levir who thus married unlawfully his brother's wife should bring a sin-offering and not, as stated in our Mishnah, an asham talui. ');">²²</span> — The other replied: Our women bear at seven months. 'Are your women', the first retorted, 'the majority of the world'! — 'What I mean', the other replied, 'is this: Most women bear at nine months and a minority at seven, and the embryo in the case of every woman who bears at nine is recognizable after a third of the period of her pregnancy;<span class="x" onmousemove="('comment',' Lit., 'her days'. ');">²³</span> and in the case of this woman, since her embryo was not recognized after a third of the period of her pregnancy<span class="x" onmousemove="('comment',' Lit., 'her days'. ');">²³</span>

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