Positive commandment · Mitzvat Aseh #100 of 613
For [a woman] who gives birth to be impure [like a woman] in the niddah state
להיות היולדת טמאה כנידה
The verse · Vayikra 12:4
וּשְׁלֹשִׁים יוֹם וּשְׁלֹשֶׁת יָמִים תֵּשֵׁב בִּדְמֵי טׇהֳרָהֿ בְּכׇל־קֹדֶשׁ לֹא־תִגָּע וְאֶל־הַמִּקְדָּשׁ לֹא תָבֹא עַד־מְלֹאת יְמֵי טׇהֳרָהּ
“And she shall then continue in the blood of her purifying for thirty three days: she shall touch no hallowed thing, nor come in to the sanctuary, until the days of her purifying are fulfilled.”
The Rambam’s words
For [a woman] who gives birth to be impure [like a woman] in the niddah state.
להיות היולדת טמאה כנידה.
Why · Sefer HaChinuch #166
Matter of the impurity of a woman who has given birth
“From the roots of the commandment, [there is a need] for a preface: There is no doubt that all of the sicknesses of people come either because of an excess in the body or from the angle of a deficit that happens from whatever reason it might be.”
Who & when
“And this commandment is practiced in every place and at all times regarding that women giving birth are impure.”
Where in the Mishneh Torah
Hilchot Those Who Defile Bed or Seat הלכות מטמאי משכב ומושב