Diaphragms are held in place by the walls of the vaginal canal, and each woman is a bit different internally. A correctly sized diaphragm will hold securely over the cervix. It will not move out of place during sex. And of course, it will be comfortable.
Not necesarily. A petite woman could actually have a larger vaginal tract than a woman twice her size or weight. And then after childbirth things would change again.
In fact, even after gaining or loosing a few kilos there's a chance that enough body fat in the lower abdomen, buttocks, inner thigh area has shifted around and increased/decreased that there could now be more, or less, pressure on the outer walls of the vagina, changing the optimum size of diaphragm thanks to a shift of just a few millimeteres.
A trained gynecologist, midwife, or healthcare practitioner will measure you for size. They will use a fitting kit with sample size diaphragms to ascertain which diaphragm will be the most effective and secure choice for you.
After your "fitting session" they'll tell you what size is correct.
The round diaphragm is folded into a cylindrical shape and then inserted into the vaginal canal. It will natural unfold inside your vaginal canal, and is then further pushed into place to cover the cervix. While the process might seem a bit daunting, with just a bit of practice most women find it simple and easy to do.
When you go for your diaphragm fitting session, the Doctor, Gyno' or Midwife who is doing it will be able to coach you on correct insertion AND removal of your diaphragm.
As an ethical company which champions women's health issues, we highly recommend consulting a health professional whenever you concider changing your chosen method of contraception.