Siman 190, the chapter on stains, is built from many details that affect one another. Let's pull the main halachot together, from the foundation through to the practical criteria, so you have one clear picture.

1. By Torah law: hargashah plus blood

By Torah law, a woman becomes a niddah only if she experienced a halachically significant hargashah (see the article on hargashah) and sees any quantity of blood. The Sages then decreed that even without a hargashah, a woman who finds a bloodstain becomes a niddah. The logic of the decree: the woman may not have noticed the hargashah, but it occurred.

2. The six criteria of a stain that renders a woman temei'ah

When the Sages enacted the decree of stains, they built several conditions into it. Not every stain renders a woman temei'ah — only one that meets all the criteria:

a. Size

The stain is larger than a gris (see the article on the gris).

b. Color

The stain is an impure color: red, black, pink, some oranges, and some shades of brown (see the article on colors).

c. A white surface

The stain is on a white surface. A stain on a colored surface does not render a woman temei'ah.

d. A surface that can receive tum'ah

The stain is on a surface that can receive tum'ah (see the article on receiving tum'ah).

e. Location

The stain is in a place the blood could plausibly have reached from the uterus. A woman who finds a stain on her shoulder, for instance, generally has no concern of being a niddah. (The exception: if she did gymnastics or the like.)

f. No alternative source

The stain cannot be attributed to a non-uterine source. If a woman finds a stain but knows she has a bleeding wound the stain can be attributed to, the stain is generally attributed to the wound and she is not a niddah. This leniency depends on several variables, so halachic guidance is preferable.

3. Heavy staining — stain or bleeding?

Sometimes a woman has heavy staining and it is unclear whether this is a halachic stain or halachic bleeding. The distinction is critical, because the six leniencies above apply only to stains, not to bleeding.

An example: a menstruating woman cannot say, "I am not a niddah, because I wore a black pad and black clothing, so all the blood was on a colored surface." If she is genuinely bleeding rather than staining, we assume she had a hargashah even if she did not notice it. A woman who saw a bloodstain on black underwear is in a different position: she is not a niddah.

Two criteria for the distinction

Two factors are weighed in distinguishing a halachic stain from halachic bleeding.

a. Hargashah — if the woman experienced a hargashah and saw a red stain, she is a niddah even if it is only a stain, regardless of size or surface.

b. The quantity of blood — here the poskim diverge widely:

Since no single answer fits every case, a woman who is unsure whether she is dealing with bleeding or staining needs halachic guidance.

4. Three situations in which a hargashah is assumed

The Gemara details several circumstances in which we assume a woman experienced a hargashah, even if she says she noticed nothing:

a. Performing a bedikah

If she performed a bedikah.

b. Marital relations

If she had marital relations.

c. After urination

If she urinated. (This is subject to an Ashkenazi–Sephardi dispute.)

The concern in each case is that the sensations accompanying these acts may have masked the hargashah.

The practical upshot