Tosefta su Isaia 26:18

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."