Midrash su Levitico 15:19

1) (Vayikra 15:19) ("And a woman, if she will have a flow, (if) blood shall be her flow in her flesh, seven days shall she be in her niddah state, and whoever touches her shall be unclean until the evening.") "And a woman": This tells me only of a woman. Whence do we derive that even a one-day-old child (is included in this context of niddah)? From "And a woman" — whence they ruled: A one-day-old child for niddah; a ten-day-old child for zivah (seven days of niddah flow and three additional days of zivah flow).

1) I might think that if she sees (a flow) three (consecutive days) in the beginning (i.e., before the onset of her niddah time) she becomes a zavah (to count seven clean days and to bring an offering). And how will I understand (Vayikra 15:19) "And a woman, if she has a flow, etc." (where seven clean days and an offering are not required)? As referring to (her seeing for) one day; but if she sees for three days in the beginning (as above) she would become a zavah (for counting clean days and for an offering). It is, therefore written (Vayikra 15 verse 25) "above her niddah state" — She becomes tamei (for the above purposes) after her niddah state, and not in the beginning (i.e., before the onset of her niddah time.)

2) "if she will have": from the pronouncement on. "a flow": I might think even if she flows from any place she is tamei; it is, therefore, written (Vayikra 20:18) "and she has revealed the source of her blood." This teaches us about (her) blood that (it causes uncleanliness) only if it comes from the source.