Niddah, Chapter 3, Mishnah 6. This chapter deals with a woman who miscarries and asks when the tumah of childbirth applies and how she counts her days of tumah and her days of taharah. Our mishnah addresses two separate questions: what happens when the gender of the child cannot be established, and at what precise moment a birth is considered to have taken place.
A tumtum and an androgynos
The mishnah begins: "Hamapeles tumtum ve'androgynos." With a tumtum, nothing is visible that would mark the child as a boy or as a girl, so the gender simply cannot be determined. An androgynos, by contrast, presents the signs of the two genders at once. Either way the ruling is "teshev lezachar velenekeivah": the tumah of childbirth applies, and since there is no way to decide which of the two sets of counts governs her, she is bound by the strict side of each. In practice she keeps fourteen days of tumah, following the pattern of a daughter, and afterward no more than twenty six days of taharah, so that the entire period closes on the fortieth day, following the pattern of a son.
Twins
The mishnah now turns to a woman who miscarried two children. "Tumtum vezachar" or an androgynos together with a boy: here too "teshev lezachar velenekeivah." Since one of the two might have been a girl, she cannot be granted the leniencies of a male birth alone, and she is again left with fourteen days of tumah followed by twenty six days of taharah.
But "tumtum unekeivah," a tumtum or an androgynos that emerged together with a girl, is a different matter: "teshev lenekeivah bilvad," she observes only the counts of a female birth. Here there is a girl before us with certainty, so she keeps fourteen days of tumah followed by sixty six days of taharah, a full eighty days in all. Fourteen and sixty six fit together perfectly, since a daughter was definitely born. And the presence of the tumtum or the androgynos alongside that daughter does not take away from her, nor does it reduce her eligibility for the full sixty six days of taharah.
When is it considered a birth?
The mishnah's final section defines the moment at which the tumah of childbirth begins. "Yatza mechutach o mesuras": if what emerged came out in pieces rather than whole, then "misheyatza rubo, harei hu keyalud," once the majority of those pieces has emerged, it is regarded as a completed birth, and from that moment she is temeiah on account of childbirth.
"Yatza kedarko": where the child came out in the normal fashion, intact and head first, the decisive moment is "ad sheyeitzei rov rosho": the point at which most of the head has come forth. Before that stage she is still tehorah; from that stage onward the tumah of a woman who has given birth rests upon her.
The mishnah closes by defining that measure: "Ve'eizeh rov rosho? Misheteitzei padachto." The majority of the head is considered to have emerged once the forehead has come out.