TT #12 · Parshas Tazria · Aliyah 1 · Passuk 2
English: Speak to the Israelite people thus: When a woman at childbirth bears a male, she shall be impure seven days; she shall be impure as at the time of her condition of menstrual separation.
Chabad.org — Chumash, English translation + Rashi for Vayikra 12:2
Torah Temimah cites Niddah 37b.
English: “As in the days of the menstruation of her sickness she shall be impure” compares the days after childbirth to menstrual days: just as menstrual days are not suitable for zavah counting and a seven-day count does not accrue from them, so too the days after childbirth are not suitable for zavah status and a seven-day count does not accrue from them.
Analogy
The niddah comparison carries over the exclusion of these days from zavah status and from a separate seven-day clean count.