A latex All-Flex arcing spring diaphragm was best for under-thrusting
An under-thrust study: A few years ago I participated in an under-thrusting study while wearing correctly sized [rather than a size or two smaller] and properly positioned diaphragms of all the available rim styles – Arcing, coil and flat spring with both uncut partners of very large girth and more normally equipped men. With the latex All-Flex (the easiest rim style to under-thrust) for the large girth men it was uncomfortable for both of us. Those encounters were part of a study my women’s clinic conducted to see which rim style put the wearer at the greatest risk of having her diaphragm accidentally under-thrust while using it for contraception during vanilla sex. All the women participating in the trial had deep post-pubic vaults as most women do. women with shallow or no post-pubic vaults aren’t good candidates for effective diaphragm use because the anterior rim is positioned where it may hurt a partner during vanilla sex and is easier to under-thrust than if the woman had a pronounced/deep post-pubic vault.
The results of the study showed that the easiest to under-thrust was All-Flex (arcing), then Milex Omniflex (coil) and Semina (coil) of similar ease. Then Milex Arcing and the most difficult was Reflexions flat spring. The Milex Arcing is more difficult than the All-Flex because it folds with two elbow hinges so the rim has to be positioned exactly right for a thrusting penis to cause it to fold. The switch from latex to silicone has not changed the study results. Of course no diaphragm is risk free since acute angle brute force thrusting can stretch the vaginal wall to allow a penis under the rim even of a Reflexions flat spring.
Penetration depth: In missionary penetration depth can depend on the shape of the couple’s pubic bones making standard missionary unlikely to attain deep thrusting unless the male partner is very well endowed. During the trial when I was taken in missionary all of my uncut partners of very large girth came away from their encounters with sore or sometimes raw frenulums. One was in so much pain he lost focus and couldn’t finish. With one or both partners having prominent pubic bones it may be necessary for the woman to put her legs on her partner’s shoulders to give him the depth to reach her cervix and the thrust angle needed for under-thrusting. I have a prominent mons veneris and at that angle I had to use lubricating gel on my labia to prevent perineal tears that less experienced women in the trial were having. It was a tiring and not all that pleasant experience.
Doggie style was somewhat better - wasn't nearly as traumatic for the men, but for me having the rim forced into my posterior fornix when he’s trying to get under it was still uncomfortable. While still tight there was more room between the back of my pubic bone and the rim (because on my elbows and knees my uterus and cervix are pulled deeper into me by gravity) for him to get the smooth top of his glans under it. And of course because of the gravity effect I could take a longer man more comfortably. Doggie is certainly the preferred position with a well endowed partner because his frenulum caresses my G-spot while his tip thrusts into my cervix.
Again the above results were for large girth men with the woman wearing a correctly sized diaphragm. For a man of more normal size the order of ease to under-thrust didn’t change, but the men suffered less physical damage. With a too small diaphragm you get all the pleasures of under-thrusting while lessening the discomfort and possibility of damage to your male partner(s). The only down-side of using a too small D is that for some women it may be difficult to remove afterward.