The Onset of the Niddah Status
A woman enters the halachic status of niddah when she experiences uterine bleeding not due to trauma.
While the most common cause of niddah is menstruation, niddah and menstruation are not synonymous. Any uterine bleeding not due to trauma (e.g., spotting caused by hormonal fluctuations, or bleeding after stopping the active pills when using combination oral contraceptives) can render a woman niddah, even when it is not menstrual. A woman can become niddah if just a drop of blood exits her uterus, at any point in her cycle..
According to Torah law, uterine bleeding renders a woman niddah only if it is one of four specific hues of red, or if it is black, and only if it is accompanied by a physical sensation (hargashah). But the expertise required to distinguish between different shades of red was lost in Talmudic times, and today any hue of red, pink, or black, as well as shades of brown that tend towards them, make a woman niddah.
A woman who discovers an apparent bloodstain on her clothing or another surface may be niddah as well. The halacha in such cases may depend on whether or not she felt a hargashah. If she did, then she is considered niddah (see Hargashah); if not, then her status depends on several factors (see Stains).
In any case in which a woman is unsure of her status, as when she has bleeding of a questionable color or finds an unexpected bloodstain, she should not independently assume that she is niddah. Rather, she should consult a halachic authority. Until her status has been clarified, however, she should follow all the laws of niddah.
Bleeding from a cut, wound, or abrasion (a makkah in halachic terms) does not, in principle, render a woman niddah, although in practice the cause of bleeding is sometimes difficult to determine and may require consultation with a halachic authority. By rabbinic enactment, many of the laws of niddah also apply to a virgin after her first intercourse (dam betulim), even though the stretching of the hymen is definitely a makkah.
There is also a halachic principle "ein petichat hamakor b'lo dam", that is, opening of the uterus is presumed to be accompanied by bleeding. Therefore, significant dilation of the uterus with an instrument can cause a woman to become niddah even if no bleeding is detected. There is debate among halachic decisors as to the minimum size of dilation for this halachic presumption to apply (opinions range from four to twenty millimeters). We follow the opinion that a dilation of 19 mm or more renders a woman niddah. Most routine gynecological procedures do not involve this degree of dilation.
A woman ceases to be niddah – and returns to a state of ritual purity (taharah) – by confirming that bleeding has ceased (hefsek taharah), counting seven blood-free days (shivah neki'im), and immersing in a proper mikveh.
The laws of niddah appear in the Torah twice. Leviticus 15:19 states: "A woman who has a flow of blood in her body shall be a "niddah" for seven days, and all who touch her shall be ritually impure until sundown." Leviticus 18:19 states: "A woman in the ritually impure state of niddah, you shall not approach for sexual relations." The first verse refers to the laws of ritual impurity (tumah v'taharah), most of which are no longer applicable today. The second verse, however, appears in the list of the most severely forbidden sexual relationships, such as adultery and incest, which remain fully relevant to this day. The practical laws of niddah are presented throughout this site.
Technically, the Torah defines a menstrual-type flow outside the usual time period or for longer than seven days as zavah (Leviticus 15:25). Nowadays, there are no practical distinctions between niddah and zavah, and in common usage, including this website, the term niddah may refer to either status.