Assumption of dam niddah

Hi,

The Talmud (Keritot 10a) states a rule, "The womb does not open without blood emerging," such that a woman whose uterus opens must assume that she is now a niddah. However, this is only if the opening is the width of a finger, at a minimum. Therefore, entry of a medical instrument does not render a woman a niddah, if the opening is that narrow - and certainly if leniency would make procreation more possible.

(Rav Moshe Feinstein, Igrot Moshe Yoreh Deah 1:89)

Have a great day,
Mordechai