For the source text click/tap here: Yevamot 42
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Our Daf determined that in order to clarify the paternity of a child which might be born, a woman must wait three months after the death of her husband before remarrying.
The Gemara then analyzes why a three-month period is necessary in order to satisfy this question.
It is clear that one month is not enough, because if a child will be born seven months later, we would not know if it was an eight-month child of the first husband or s seven-month child of the second husband.
The Gemara probes, however, to determine why waiting two and a half months would not be sufficient.
We explore the ancient cross cultural notion of viability of fetus at 7month but not 8 months.
How this carries into modern day halachic determinations.