A diaphragm is a shallow, dome-shaped cup made of silicone that you insert into the vagina before sex to physically block sperm from reaching the cervix. Used with spermicide and placed correctly every time, it prevents pregnancy about 94% of the time. With typical use, that number drops to about 88%, meaning roughly 12 out of 100 women using a diaphragm will become pregnant in a year.
How It Creates a Barrier
The diaphragm works by covering the cervix, the narrow opening at the bottom of the uterus where sperm need to pass through to reach an egg. When the flexible rim is positioned correctly, it tucks behind the pubic bone in front and sits snugly against the back wall of the vaginal canal, forming a seal around the cervix. This physical barrier alone isn’t the whole story, though.
Spermicide, a cream or jelly applied to the inside of the dome before insertion, acts as a chemical backup. It kills or immobilizes any sperm that manage to slip past the rim. The combination of physical blockade and spermicide is what gives the diaphragm its effectiveness. Without spermicide, the failure rate climbs significantly, which is why every set of usage instructions treats them as a pair.
Insertion and Timing
You can insert the diaphragm up to 2 hours before vaginal sex. To place it, you fold the flexible rim in half, slide it into the vagina, and push it toward the back wall until the far edge sits deep behind the cervix. Then you nudge the front rim up so it tucks directly behind the pubic bone. You should be able to feel your cervix through the dome, which confirms it’s in the right spot.
After sex, the diaphragm needs to stay in place for at least 6 hours. This waiting period gives the spermicide time to neutralize any remaining sperm. If you have sex again during that window, you add more spermicide without removing the diaphragm.
Fitting and Sizing
Traditional diaphragms come in sizes ranging from 60 to 90 millimeters in diameter, and getting the right fit matters. Too small, and it can shift during sex. Too large, and it’s uncomfortable or folds in on itself. A healthcare provider determines your size by measuring the distance between the back of the vaginal canal and the pubic bone during a pelvic exam, then matching that measurement to a fitting ring.
A newer option called Caya is a contoured, single-size diaphragm made from non-latex silicone. It fits most people who would have been sized between 65mm and 80mm in a traditional diaphragm, which covers the majority of users. Caya still requires a prescription, and if you’ve never used a diaphragm before, a provider visit to confirm proper placement is recommended. Many of the older multi-size brands have been discontinued, though the Milex Wide Seal remains available in the U.S. through a clinician’s office.
How Effective It Really Is
CDC data puts the typical-use failure rate at 12% per year and the perfect-use failure rate at 6%. The gap between those numbers comes down to human error: forgetting to apply spermicide, inserting the diaphragm in a rush and not checking cervical coverage, or removing it too soon after sex. Only about 57% of diaphragm users are still using the method after one year, which reflects how sensitive it is to consistent, correct technique.
For comparison, hormonal IUDs have a typical-use failure rate below 1%, and male condoms sit around 13%. The diaphragm lands in a middle range: more effective than no method, less effective than long-acting options, and heavily dependent on how carefully you use it each time.
The UTI Connection
One well-documented downside of diaphragm use is an increased risk of urinary tract infections. A study published in JAMA found that diaphragm users developed UTIs at roughly three times the rate of women on oral contraceptives: 26.6 infections per 1,000 patient-months compared to 8.9. The increased risk couldn’t be explained by differences in age, sexual activity, or UTI history.
The likely cause is mechanical. The rim of the diaphragm presses against the urethra and can slow urine flow, making it easier for bacteria to colonize the urinary tract. If you’re someone who already gets frequent UTIs, a diaphragm may not be the best fit for you. The NHS lists recurrent UTIs as a reason to consider a different method.
Who Shouldn’t Use One
Beyond UTI risk, there are a few situations where a diaphragm isn’t appropriate. You should avoid it if you have a latex allergy (though newer silicone options like Caya sidestep this), if you’ve ever had toxic shock syndrome, or if you or a partner currently has an untreated sexually transmitted infection. People at higher risk of HIV are also advised to choose a different method, since the diaphragm doesn’t protect against STIs the way condoms do.
Childbirth changes vaginal anatomy, which means a diaphragm fitted before pregnancy likely won’t fit afterward. You’ll need to be re-sized after giving birth, and cap-style devices in particular become less effective for people who have delivered vaginally.
Care and Replacement
After each use, wash the diaphragm with mild soap and warm water, let it air dry, and store it in its case. Avoid oil-based lubricants or vaginal medications, which can degrade silicone over time. Before each use, hold it up to the light and check for thin spots, holes, or cracks in the material. A well-maintained silicone diaphragm typically lasts one to two years before it should be replaced, though your provider can give more specific guidance based on the brand you’re using.