Short answer: Jainism does not call a menstruating woman “sinful.” The monthly cycle is a natural bodily process. What tradition describes as “ashaucha” (ritual non‑sanctity) applies to all bodily discharges—blood, semen, urine, feces, saliva, etc.—because they involve countless tiny living organisms and uncleanliness. To protect ahimsa and the sanctity of worship, certain temple and idol‑touching activities are paused for a few days.

Why it is treated as ritually impure (ashaucha)

• Ahimsa: Menstrual blood and related secretions carry innumerable micro‑life forms. Handling idols, preparing offerings, or elaborate rituals may increase inadvertent himsa.
• Saucha (cleanliness) for worship: Touching the Jina idol and performing abhisheka require the highest physical and mental purity; contact with any bodily discharge temporarily disqualifies one from those acts.
• Care of the body: Tradition also allows rest and reduced ritual load during those days.

What is generally advised

• Avoid for a few days: entering the sanctum (garbhagriha), touching idols, abhisheka, preparing temple naivedya, or taking certain vows/fasts tied to idol‑worship.
• Allowed and encouraged at home: svadhyaya (study), japa, contemplation, listening to pravachans, and gentle samayika without touching idols or scriptures if your household achar says so. The person is not considered morally impure; only certain touch‑based temple rites are paused.

Differences in traditions

• Shvetambar: Common practice is to refrain from puja/abhisheka and entering the sanctum during menses; mental worship, bhavana, listening to discourses, and study are fine at home.
• Digambar: Similar caution—no idol‑touching or temple rituals; home‑based svadhyaya and meditation are acceptable.

Customs can vary by sangh/gachchha and family achar; follow your community’s achar‑grantha guidance.

Essence

• Menstruation itself is not a sin. The “impurity” is ritual/functional, meant to uphold ahimsa and the exceptional cleanliness expected for idol‑centered rites. Compassion, modesty, and non‑violence remain the focus, never stigma or shame.