Niddah, Chapter 3, Mishnah 4. This chapter examines the woman who miscarries and asks in each case what status the miscarriage leaves her in: is she temeiah as a niddah, is she temeiah as a yoledes (a woman who has given birth), or is she not temeiah at all? Everything depends on what emerged from her. Our mishnah addresses a sac or membrane that came out of her, and distinguishes between one that is empty of any fetus and one that contains a formed child.
A Sac With No Child In It
The mishnah begins: "Hamapeles shaprir," a woman who expels a shaprir, which is a membrane or sac. Three possibilities are listed for what such a sac may hold. It may be "malei mayim," its contents being water; "malei dam," its contents being blood; or "malei genunim," its contents being little bits of flesh. Regarding all three the ruling is "einah chosheshes levlad": there is no reason to suspect that a child was ever present here. Consequently no tumah attaches to her at all, neither the tumah of a niddah nor that of a woman who has given birth.
When the Sac Contains a Formed Fetus
"Ve'im hayah merukam": if, however, the sac was merukam, meaning that a formed fetus can be seen within it, the picture changes entirely. Now she does become temeiah with tumas leidah, the tumah of a woman who has given birth.
The mishnah rules "teishev lezachar velanekeivah": since the fetus is inside the membrane and it cannot be established whether it is male or female, she must observe the stringencies of both a male birth and a female birth. To appreciate what that means, recall the two counts. For a male child a woman has seven days of tumah followed by thirty-three days of taharah, forty days in all. For a female child she has fourteen days of tumah followed by sixty-six days of taharah, eighty days in all.
Combining the stringencies of both yields fourteen days of tumah, the longer tumah period that belongs to a female, followed by twenty-six days of taharah, so that her days of taharah come to an end on the fortieth day, as they would for a male. Fourteen plus twenty-six equals forty, and from that point on she can no longer rely on the leniency of the days of taharah.