2,134. Thus Far Has Been Biblical Law...
Hilchos Issurei Biah 11:1
Everything that has been said about a niddah, a zavah and a woman who has given birth is according to Biblical law. Formerly, these procedures were observed when the Sanhedrin was in session, as it included great scholars who were proficient in the intricacies of blood. If there was a doubt about whether blood was found or about niddah days and zivah days, one could go to the Sanhedrin and ask as per Deuteronomy 17:8, “If there arises a matter too hard for you in judgment, between blood and blood, between law and law (…then go up to the place that Hashem your God will select).” “Between blood and blood” means differentiating between niddah blood and zivah blood. In those days, women were conscientious in this matter; they would pay attention to their monthly cycles and would scrupulously count their niddah days and their zivah days.
Hilchos Issurei Biah 11:2
It takes a lot of effort to count the days and cases of doubt will frequently arise. Even if a woman saw blood on the day she was born, she must start counting her niddah days and zivah days as was discussed (in chapter 4). Accordingly, a girl can’t be a zavah until she’s ten days old because if she saw blood on the day that she was born, she’d be a niddah for seven days, plus (she’d need to see blood) for the three days immediately after her niddah days, which makes ten days. Therefore, a woman starts counting her niddah days and zivah days for her entire life starting from the first time she sees blood, even if that’s as a minor.