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I normally tell people not to pay attention to the size suggestions because so many people can switch out between the two sizes no matter if they had children or not and no matter what age they are.
Does your daughter know the approximate height of her cervix? If it’s low, I would suggest a bell shaped cup like Super Jennie, Si-Bell, Lena, LaliCup, Fleur Cup. All of these will allow a low cervix to sit inside of the cup without taking up room.
If she has a medium/high cervix, those cups still might work great but she’ll probably be just fine trying any cup in a large size, as well.
My daughter has not had children yet but she still wears all of the large sized cups comfortably and normally uses a Super Jennie large for the capacity.
So basically, you’re choosing a cup to fit your (daughters) needs and not so much on what the guidelines suggest. The guidelines are great for a first timer that doesn’t know where to begin. However, since we know she’s already comfortable inserting and using a menstrual cup, she’ll be more confident with trying the larger size.
I agree with Red, she can try a larger size since the small one leaks, it’s probably just too small, age is not the only factor, I know some women that had 3 kids that still wear a small size.
Have a look at my compare table and sort the diameter and length to give you an idea. Look for example here I sorted the biggest diameter first. You can see then how long they are and trying to find the best combination that suits your daughter’s body.
I also have a heavy flow, but I’ve tried the smaller size of diva cup, and it was too large to comfortably and easily unfold and create a seal. The small size of meluna cup seemed to fit and create a seal, but I have to change it almost hourly on the really heavy days.
Based on my experience with a larger size cup than I have now, I’m doubtful that something that fits my heavy flow with necessarily fit my vagina.