Commentaire sur Nida 3:6

ואין ידוע מה הוא – if it is a male or a female.

If a woman miscarried and it is unknown what it was, she sits for both a male child and a female child. This is the same rule we have learned in previous mishnayot.

תשב לזכר ולנקבה – the days of purity of a male, and the days of [ritual] impurity of a female.

If it is unknown whether it was a child or not, she sits for both a male and a female and as a menstruant. In this case, she isn't even sure if she miscarried or whether she menstruated. The rule is a bit more complicated here because she must also observe the laws of menstrual purity. This means that there will be no days of pure blood, as there would be if she had given birth to either a male or female. The first 14 days after the event, she will be impure as if she had given birth to a female. If she sees blood immediately after that, it is considered menstrual blood, because this is more than 11 days after her previous blood. If she sees blood on the 34th day, the rule is a bit complicated, for in this case we must be concerned that she had given birth to a male. If she sees blood on the 34th day and then again on the 41st day, if we considered her as a niddah at the outset, then at the time of the blood on the 41st she would not be in niddah, because it is within 11 days of the previous blood (on the 34th day), and she would observe only one day of impurity for every day she sees blood (up to three days). If she had given birth to a girl, then she would pure throughout the whole period (she would be pure from days 15-80). However, if she gave birth to a male, then the blood on the 34th day would be pure because it was within the 33 days of purity after the week of impurity. The blood on the 41st day would be menstrual blood because it was more than 11 days after he previous menstrual blood. Therefore, in this case she must be concerned that the blood is menstrual and she would have to observe the rules of menstrual impurity.

לזכר ונקבה ולנדה – meaning to say, that when you see that she is a menstruant woman, that she doesn’t have the days of purity, for perhaps it is not a fetus. And she immerses [in a Mikveh] at the conclusion of forty days like when she gives birth to a male [child] according to the one who states that ritual immersion at its appropriate time is a Mitzvah. And she immerses a long day, she needs another ritual -immersion at the end of the fortieth [day], and this is needed for immersion at the conclusion of forty days like one who gives birth to a male [child], even though this immersion does not have any effect upon her whatsoever, for she is not pure for Holy Things and for the Temple until her fulfillment of eighty days like giving birth to a female [child],