פירוש על נדה 3:7

המפלת ליום ארבעים – of her ritual immersion.

If a woman miscarried on the fortieth day, she need not be concerned that it was a valid childbirth. The rabbis believed that it took forty days for a fetus to form. We should note that this was a common belief in the ancient world, one that I believe was shared by Aristotle as well. [It also had implications for the laws concerning abortion. For instance Philo, an early Jewish philosopher, stated that abortion was permitted up till the fortieth day, because that is when the fetus is formed.] If she miscarries before the fortieth day, the rabbis believe that it could not be a fetus.

אינה חוששת לולד – that she did not cause his formation until the completion of all forty days.

On the forty-first day, she sits as for both a male and a female and as for a menstruant. If she miscarries on the 41st day, and it is not clear if it was a miscarriage or blood, she must be concerned lest it was a male, lest it was a female and lest it was menstruation. This situation was explained in yesterday's mishnah.

ולנדה - when she sees [blood], she will be a menstruant woman, for perhaps there isn’t a [human] fetus [after all], and there aren’t the days of purity.

Rabbi Ishmael says: [if she miscarried on] the forty-first day she sits as for a male and as for a menstruant, But if on the eighty-first day she sits as for a male and a female and a menstruant, because a male is fully fashioned on the forty-first day and a female on the eighty-first day. Rabbi Ishmael seems to believe that males are formed on the 41st day after conception and females on the 81st day. Therefore, if she miscarries between the 41st and 80th days, she need not be concerned lest it was female. [What he is saying is that females begin as males and only turn into females on day 81. Interesting, although obviously incorrect]. This means that she will be impure for seven days. If blood appears after this time, she is again a niddah. The blood is not pure because we are not sure if she gave birth.

יום ארבעים ואחד תשב לזכר – seven days of impurity. Even with her giving birth dried up/withered she is not a menstruant woman. But two weeks [after giving birth to] a female [child], she does not sit, as the reason is taught that the formation of a female is [after] eighty-one days. But the Halakha is not according to Rabbi Yishmael.

But the sages say: the fashioning of the male and the fashioning of the female both take forty-one days. The sages espouse a more egalitarian view both males and females are formed on the 41st day. If she miscarries after this, she must be concerned lest it was a male, female or menstruation, as we explained above.

וחכמים אומרים כו' – The Sages are the first Tanna/teacher, but because that the reason of Rabbi Ishmael makes more sense than the first Tanna/teacher, Rabbi [Judah the Prince] retracted and taught it in the language of the Sages, to teach us that the Halakha is according to the first anonymous teaching, and even though there is the dispute of Rabbi Yishmael at its side.