Mishnayos Niddah Perek 3 Mishnah 7

A woman who discharges on the fortieth day since she immersed herself and engaged in intercourse with her husband need not be concerned that it might have been an offspring and she became impure with its miscarriage, as the formation of the offspring in the womb occurs only forty days after conception. But in the case of a woman who discharges on the forty-first day after immersion, there is concern that perhaps it was an offspring. Since its sex is unknown, she shall observe the period of impurity for a woman who gave birth to a male and for a woman who gave birth to a female; and for any blood that she sees, she observes the halakhot of a menstruating woman. Rabbi Yishmael says: A woman who discharges on the forty-first day after immersion observes the seven days of impurity for a woman who gave birth to a male; and for any blood that she sees after seven days, she observes the halakhot of a menstruating woman. But a woman who discharges on the eighty-first day after immersion observes the strictures of a woman who gave birth both to a male and to a female, and also the strictures of a menstruating woman, as the formation of the male offspring concludes on the forty-first day and the formation of the female offspring concludes on the eighty-first day. And the Rabbis say: With regard to both the formation of the male and the formation of the female, this and that conclude on the forty-first day.

הַמַּפֶּלֶת לְיוֹם אַרְבָּעִים, אֵינָהּ חוֹשֶׁשֶׁת לְוָלָד. לְיוֹם אַרְבָּעִים וְאֶחָד, תֵּשֵׁב לְזָכָר וְלִנְקֵבָה וּלְנִדָּה. רַבִּי יִשְׁמָעֵאל אוֹמֵר, יוֹם אַרְבָּעִים וְאֶחָד, תֵּשֵׁב לְזָכָר וּלְנִדָּה. יוֹם שְׁמוֹנִים וְאֶחָד, תֵּשֵׁב לְזָכָר וְלִנְקֵבָה וּלְנִדָּה, שֶׁהַזָּכָר נִגְמָר לְאַרְבָּעִים וְאֶחָד, וְהַנְּקֵבָה לִשְׁמוֹנִים וְאֶחָד. וַחֲכָמִים אוֹמְרִים, אֶחָד בְּרִיַּת הַזָּכָר וְאֶחָד בְּרִיַּת הַנְּקֵבָה, זֶה וָזֶה לְאַרְבָּעִים וְאֶחָד: