The Mishnah in Masechet Niddah (59b) records a machloket about a woman who urinated and found blood in the vessel she urinated into.

The dispute between Rabbi Meir and Rabbi Yosei

Rabbi Meir says we must distinguish between a woman who was sitting while urinating and one who was standing:

Rabbi Yosei holds that whether she sat or stood, she is not a niddah.

The Gemara's explanation

The Gemara explains Rabbi Meir's position. If the woman was sitting, any blood found clearly came from the urinary tract rather than the uterus. If she was standing, we are concerned that the urine flowed backward toward the uterus and carried uterine blood with it.

The Gemara qualifies that Rabbi Meir's position regarding a woman who sat holds only with two further conditions:

  1. She sat on the rim of the vessel.
  2. The urine came down in a continuous stream.

The halachic ruling

The Gemara makes clear that the halachah follows Rabbi Yosei. The Rishonim divide over what exactly Rabbi Yosei meant.

According to the Rambam

Rabbi Yosei holds that in every case, a woman who urinated and found blood in the vessel is not a niddah.

According to the Rosh

Rabbi Yosei required chada l'tivuta — one favorable condition to rule her tehorah: either that she sat, or that she stood and the urine came down in a continuous stream. With neither condition met, she is a niddah.

According to Rabbeinu Chananel

Rabbi Yosei required four conditions to rule her tehorah:

  1. She sat.
  2. She sat on the rim of the vessel.
  3. The urine came down in a continuous stream.
  4. The blood was found mixed with the urine rather than on the wall of the vessel.

The rulings of the Shulchan Aruch and the Rema

The Shulchan Aruch rules like the Rambam: a woman who finds blood in her urine is not a niddah.

The Rema cites the Rosh's position but ultimately rules stringently, like Rabbeinu Chananel: only if all four conditions are met is she not a niddah.

Practical halachah today

In practice, an Ashkenazi woman who finds blood after urinating does not work through the four conditions one by one, for several reasons.

The first reason — the loss of expertise

The Sidrei Taharah writes that we are no longer expert in defining a continuous stream, so that condition cannot be assessed.

The second reason — the change in surroundings

Women today do not use chamber pots, which could receive tum'ah. Toilets are attached to the ground and cannot.