Hello, there are different IUDs requiring different sizes of uterine opening during the insertion. I read somewhere that any opening |4mm would render a woman niddah. Other sources say that a uterine opening up to 19mm would be okay. Which one is correct? Based on which ruling? Thank you for your help!

Shalom, As you stated, there are diferent opinions as to what is the minimum opening of the crevix during a medical procedure that would render a woman niddah. Rabbi Moshe Feinstein was lenient up to 19mm, while others, such as Rabbi Wosner, were strict even for an opening of 4mm. Each person should follow their own Rabbi's instruction. Many follow the lenient ruling of Rabbi Feinstein. Many women experience bleeding as result of the insersion of the IUD. If it is localized minimal bleeding this can be considered an abrasion and would not make a woman niddah. However, if there was a large amount of bleeding over a longer period this may be more problematic. Kol Tuv