If the Mekor, usually defined as uterus, opened it would automatically render the women impure since there is an supposition that anytime the uterus opens blood comes out. Halacha follows that assumption.
Chazal established a principle that once the uterus opens we assume that blood must have exited and we treat the woman as tameh. Therefore, some poskim write that if a doctor performs a procedure such that the uterus was opened at least 15mm she is rendered tameh as a niddah. However, if the uterus is opened less than 15mm she is tahora.
- The Nodeh Beyehuda 2:120 writes that it is obvious to him that there is no difference whether the uterus was opened by itself or externally by a doctor with a finger or instrument, either way she is tameh. The Binat Adam 23 argues that certainly opening the uterus with just a finger doesn’t render her tameh, otherwise she could make herself tameh when she does a hefsek tahara. Alternatively, the Chatom Sofer 179 writes that it is impossible for a finger to penetrate so deeply as to open the uterus.
- Rav Ovadia Yosef in Taharat Habayit v. 2 p. 55 is lenient if she doesn’t see any blood for a combination of reasons including: (1) Some poskim (Rambam) hold that the opening of the uterus doesn’t mean she automatically will see blood. (2) The Raavad holds that the principle that once the uterus is open she will see blood is based a majority but it is possible for the blood that exits to be tahor blood. (3) Some achronim (Tiferet LMoshe 188) disagree with the Nodeh Beyehuda and thinks that the principle that she is tameh automatically when the uterus opens only applies if it opened naturally but not if it was opened by a doctor. (4) The Teshuva M’Ahava 1:116 quotes the Nodeh Beyehuda as saying that the principle that a woman would see blood when the uterus is open only applies when a solid or semi-solid item exits as well, but if liquids or nothing came out then she is not tameh just because the uterus opened.
- Badei Hashulchan 194:31 cites the dispute and is strict unless the instrument inserted was 15mm or less since that is the size of a small finger.