Ramban comments on the verse and discusses the nature of the childbirth impurity and the biblical structure of seven/thirty-three for a boy and fourteen/sixty-six for a girl. He frames the law in both peshat and symbolic terms.
וְטַעַם בִּדְמֵי טָהֳרָה עַל דַּעַת רַשִׁ"י לוֹמַר שאע"פ שֶׁרָאֲתָה טְהוֹרָה מִן הַתּוֹרָה, וְכָךְ אָמַר ר"א (אבן עזרא על ויקרא י"ב:ד'), כִּי הַטַּעַם שֶׁהוּא דַּם טֹהַר כְּנֶגֶד דַּם הַנִּדָּה וְאֵינוֹ מְטַמֵּא, וְהַשֵּׁם גָּזַר עַל הַזָּכָר כְּמִסְפַּר הַיָּמִים שֶׁתִּשְׁלַם צוּרָתוֹ בַּבֶּטֶן וְהַנְּקֵבָה כִּפְלַיִם, וְזֶה דָּבָר בָּרוּר וּמְנֻסֶּה. וּלְפִי דַּעְתִּי כִּי טַעַם טָהֳרָה - נִקָּיוֹן, כִּלְשׁוֹן "זָהָב טָהוֹר" (שמות כה לט) - צָרוּף וּמְזֻקָּק, וְכֵן "וְיָשַׁב מְצָרֵף וּמְטַהֵר כֶּסֶף וְטִהַר אֶת בְּנֵי לֵוִי וְזִקַּק אֹתָם" (מלאכי ג ג). וְהָעִנְיָן כִּי צִוָּה בְּיוֹלֶדֶת זָכָר שֶׁתִּטְמָא שִׁבְעָה כְּנִדָּתָהּ, כִּי הַמִּנְהָג שֶׁתִּהְיֶה שׁוֹפַעַת דָּם מִן הַמָּקוֹר, וְצִוָּה שֶׁתּוֹחִיל עוֹד שְׁלֹשִׁים וּשְׁלֹשָׁה יָמִים תֵּשֵׁב בְּבֵיתָהּ לְנַקּוֹת גּוּפָהּ, כִּי בְּכָל אֵלֶּה תּוֹצִיא תַּמְצִית הַדָּמִים וְהַלֵּחוֹת הָעֲכוּרוֹת הַמְּעֻפָּשׁוֹת הַבָּאוֹת מִתַּמְצִית הַדָּם, וְאָז תִּנָּקֶה מִלֵּדָה וּמִבֶּטֶן וּמֵהֵרָיוֹן וְתָבֹא בֵּית ה'. וְרַבּוֹתֵינוּ קִבְּלוּ שֶׁהִיא טְהוֹרָה לְבַעְלָהּ בַּיָּמִים הָאֵלּוּ, מִפְּנֵי שֶׁאָמַר בַּשִּׁבְעָה שֶׁהֵם כִּימֵי נִדַּת דְּוֹתָהּ, וּבְאֵלֶּה אָמַר שֶׁתִּטְמָא לַקֹּדֶשׁ וְלַמִּקְדָּשׁ, לֹא לְחֻלִּין וְלֹא לַבַּעַל, וּכְמוֹ שֶׁאָמְרוּ (חולין לא), בַּעְלָהּ חֻלִּין הוּא. וְטַעַם הַכֶּפֶל בַּנְּקֵבָה, אוֹ כְּדִבְרֵי רַבִּי אַבְרָהָם עַל דַּעַת ר' יִשְׁמָעֵאל שֶׁאָמַר שֶׁהַזָּכָר נִגְמָר לְאַרְבָּעִים וְאֶחָד יוֹם וְהַנְּקֵבָה לִשְׁמוֹנִים וְאֶחָד (נדה ל), אֲבָל לְדַעַת חֲכָמִים שֶׁאָמְרוּ אֶחָד זָכָר וְאֶחָד נְקֵבָה לְאַרְבָּעִים וְאֶחָד, הַטַּעַם בַּעֲבוּר כִּי טֶבַע הַנְּקֵבָה קַר וְלַח, וְהַלְּבָנָה בְּרֶחֶם הָאֵם רַבָּה מְאֹד וְקָרָה, וְעַל כֵּן יָלְדָה נְקֵבָה, וְעַל כֵּן צְרִיכָה נִקָּיוֹן גָּדוֹל מִפְּנֵי רִבּוּי הַלֵּחוֹת וְהַדָּם הַמְּעֻפָּשׁ שֶׁבָּהֶן וּמִפְּנֵי קְרִירוּת, כַּיָּדוּעַ כִּי הֶחֳלָיִים הַקָּרִים צְרִיכִין בִּנְקִיּוּתָם אֲרִיכוּת זְמַן יוֹתֵר מִן הַחַמִּים:
IN THE BLOOD OF ‘TAHORAH’ (PURIFICATION). According to Rashi this means that even though she sees [an issue of blood during this period of thirty-three days for a male child, and sixty-six for a female], she is nevertheless pure by law of the Torah. And so did Rabbi Abraham ibn Ezra write, that the meaning of the above expression is that “it constitutes pure blood in contrast to the blood of the menstruant, and therefore does not convey impurity, G-d having decreed in the case of a male child [a forty-days period for the after-effects of the childbirth upon the mother — seven impure days and thirty-three pure days], corresponding to the number of days necessary for the form of the male child to be completed in the womb, while that of the female child is double [fourteen impure days and sixty-six pure days, corresponding to the eighty days it takes for the female child to be formed in the mother’s womb]. This is clear and tested.” But in my opinion the meaning of the word tahorah is cleanness [in a physical sense], similar in meaning to ‘zahav tahor’ (pure gold), which means smelted and refined. A similar expression is found in the verse, And he shall sit as a refiner and purifier of silver, and he shall purify the sons of Levi, and purge them as gold and silver. Thus the meaning of the expression here is as follows. Having commanded that a woman who gives birth to a male child be impure for seven days as in the days of her impurity, because then she usually sees issues of blood from the interior of the womb [from which the menses are discharged], He further commanded that she should wait for another thirty-three days, staying in her house in order to cleanse her body; for during all these days she will emit the remnants of blood and the turbid, ill-smelling secretions which come from these bloods, and then she will become cleansed from the childbirth, pregnancy and conception, and she may come to the House of G-d. Now our Rabbis have received the tradition that during these [thirty-three days for a male child and sixty-six for a female], she is pure for her husband, because with reference to the seven impure days it says that they are as in the days of the impurity of her sickness, but in connection with these [thirty-three days etc.] He said that she is impure as regards [eating or touching] hallowed things and entering the Sanctuary, but not for non-holy things nor for her husband, just as the Rabbis have said, “Her husband is not a holy object.” The reason that the time is doubled in the case of the birth of a female child [i.e., fourteen impure days and sixty-six pure days], is perhaps as Rabbi Abraham ibn Ezra said [as mentioned above] in accordance with the opinion of Rabbi Yishmael, that the formation of a male child is completed on the forty-first day, and that of a female child on the eighty-first day. But according to the opinion of the Sages who say that both male and female are completed on the forty-first day, we must say that the reason [why the time is doubled in the case of a female child] is that the nature of the female is cold and moist, and the white [fluid] in the mother’s womb is then exceedingly abundant and cold, this being the reason why she gave birth to a female child. Hence she needs a longer time to become clean [in a physical sense], on account of the abundant moisture in her which contains the ill-smelling blood, and on account of the coldness [of her body], as is well-known that sick people who suffer from cold need a longer period to restore their vigor than those who are hot.