Just for clarity - and I'm not advocating for someone who has no real medical knowledge to perform either - but two very different procedures are being mashed together as if they are the same thing. If you are going to play in this space you really need to know the differences, and if you are getting the answers that are included in this chain then you don't have the right person....sorry that's not meant to offend but the advice here is not quite medically accurate.
Cervical dilation - this is typically accomplished with a set of Heger or other brand cervical dilators, medical laminaria, or cervical stenosis probe. It's going to cause cramping in most women, though it's different from person to person. It can be extremely pleasurable for some women which is why this comes up from time to time. Cervical dilation does not involve introducing a device into the uterine fundus to determine depth or side to side measurement. Outside of pregnancy dilation a common reason for dilation is to determine if cervical stenosis is present. Again, if your play doc is describing this procedure as "cervical sounding" or suggesting that it involves any uterine penetration beyond the cervical length you are not playing with the right person.
Uterine sounding - typically performed prior to IUD insertion. It's done to ensure depth is set to prevent perforation from the IUD insertion itself. The type of sound used typically has a rounded bulb end, sometimes endometrial aspirators/samplers are used for this purpose. Although the cervix is dilated as a result - this is not simply cervical dilation and really not a play option unless you have some sort of private agreement with a licensed physician in a real medical office setting.
Again I'm pointing this out for women considering cervical procedures as part of play, not for everyone in this group to lash out at me for setting the record straight - if your play doc talks about cervical dilation and "sounding" of your uterus in the way it's being described in this thread you are dealing with someone who has no idea of what they are doing. You need to avoid that person. They and others read stuff on the internet and paste it on here or have a conversation with you based off something they read on the internet ... that is not actual experience of performing the procedure or the knowledge required.
The warnings of being careful are well taken - but cervical dilation and uterine depth sounding is absolutely not the same procedure. In the right hands with someone who actually knows how to use a tenaculum, speculum, dilators....that person will also know about the quality and type of instruments required, how to properly clean the vagina and cervix prior to use, as well as proper care of the instruments involved. That person will also explain to you everything about the procedure and how it can integrate into play and advise you that actual medical advice cannot be conveyed verbally, in a forum such as this, or in any other manner outside of talking to a licensed professional.
Enough said - playing safe means playing informed