A woman who has her period has niddah impurity and is forbidden sexual contact with her partner.
But not only during her period is she impure; if a single drop of blood has issued from her womb she has niddah impurity.
The sages were divided about a woman who urinates and sees blood in the urine. Should she fear that the blood in the urine is menstrual blood, thereby rendering her impure, or is the blood in the urine considered blood from a wound or blow in the urinary tract which was expelled along with the urine, leaving the woman pure? According to one sage she is pure. According to another sage, it depends. If the woman urinated while she was sitting she is pure, but if while she was standing, she is impure.
The Talmudic sages asked why it makes a difference if she urinates in a seated position or standing. They answered that when a woman stands she urinates under stress; the urine is held in the vagina, rises to the womb, and pulls blood with it, so the woman is impure. But when she urinates in a seated position, she does so with no stress and urinates a steady stream; the urine is not held in the vagina or rise to the womb. The scholars then asked: even when she sits there is room for doubt lest the blood issued from her womb after she finished urinating. They answered that in this case the woman sat on the edge of the toilet with her vagina on the lip; she urinated in a stream into the toilet. If blood is found inside the toilet, this blood was expelled with the urine and the woman is considered pure. But if the blood is on the lip of the toilet one must suspect that this blood came from the womb and is therefore impure. (The commentators were divided on this issue and spilled much ink and effort on it.)
(Babylonian Talmud, Tractate Niddah 59b. The religious arbiters were divided on this issue; see Shulchan Aruch Yoreh Deah 191:1)
>b>The Talmudic text:

Mishnah: If a woman urinated and saw blood.
Rabbi Meir says: If she was standing, she is impure; if she was sitting, she is pure.
Rabbi Yosi says: In either case, she is pure.
Gemara: What difference does it make if she was standing or sitting? Surely, the stringency of standing is that the urine goes to the source of menstrual blood [before coming out], the blood came from there — also when she sits it goes to the source! Shmuel says: The case is, she allowed the urine to spurt. Should we be concerned that after the urine spurted, blood came from the source? R’ Aba says: The case is, she sat on the edge of the vessel — had the blood came after the spurting stopped, it would have been found on the vessel.