Tosefta sur Nida 1:11

Who is [considered to be] a virgin [for purposes of niddah] (see Niddah 1:3)? Any girl who has not seen blood in her days, even if she is married and she has children, I call her a virgin, until her eyes have seen the first. Do not call her a virgin with respect to virginity, rather a virgin with respect to blood. Who is [considered to be] a "pregnant woman"? Summachos says in the name of Rabbi Meir, one whose status of being pregnant is known [to those around her] for three months, as it is said (Gen. 38:24): "And about three months had passed [after which it was told to Yehuda, Tamar your daughter-in-law has played the whore and indeed, behold, has become pregnant from whoring...]." [If] there was a presumption of pregnancy, and then she saw [blood], or she miscarried, [or] afterwards she [gave birth to] something that was not a child, it suffices [to reckon here impurity from] her set time (Eduyot 1:1, Kulp tr.). And even though there is no explicit proof about the matter, there is at least a hint (Is. 26:18), "Behold, we travailed as though we had given birth to the wind."

The four women as to whom [the Sages] said "their time is sufficient" (Nidd. 1:3), [if] they saw a stain and afterwards saw blood, their time is sufficient. If they skipped over three cycles, whether they saw it or they did not see it, and afterwards they saw it, their time is sufficient. [If] they skipped over three cycles within their days of purity and afterwards saw it, their time is sufficient.