I've used a cup for years but have never had one be truly leak-free. I don't know if it's a pelvic floor muscle issue (I'm 30 and haven't given birth) and both small and large cups will have some leaks. I have a heavy flow and a low cervix. The cup obviously leaks if it's full, but later in my period when the menstrual fluid is thinner it will leak a little bit as well, before it's even close to full (and especially when I'm sitting at an angle or lying down).
i love the idea of cups, and I've used one exclusively for years, but it's just frustrating to never have one actually be leak-free. I wouldn't trust it to swim with, for example.
Any input is appreciated. :)