A diaphragm is a shallow, dome-shaped cup made of silicone that you insert into your vagina before sex to cover your cervix. It works as a physical barrier, preventing sperm from reaching your uterus. With typical use, diaphragms prevent pregnancy about 87% of the time. With perfect use every single time, that number rises to about 94%.
How a Diaphragm Prevents Pregnancy
The diaphragm sits inside your vagina and covers your cervix, which is the narrow opening between the vagina and uterus. Sperm can’t pass through the barrier to reach an egg. Diaphragms should always be paired with spermicide, a gel or cream that immobilizes or kills sperm on contact. The combination of physical barrier plus spermicide is what gives the method its effectiveness.
Without spermicide, pregnancy rates climb noticeably. One clinical study found that over 12 months, about 29 out of 100 women using a diaphragm without spermicide became pregnant under typical use, compared to about 21 out of 100 using one with spermicide. Among women who used the method consistently and correctly, those numbers dropped to roughly 19 and 12 per 100, respectively. The takeaway: spermicide meaningfully improves the diaphragm’s reliability, and consistent correct use matters even more.
Traditional vs. Modern One-Size Design
Older diaphragms came in up to nine sizes, ranging from 55 mm to 95 mm in diameter, with different spring styles. Getting one required a pelvic exam where a provider estimated the distance between internal landmarks in your vagina and fitted the largest size you could comfortably wear. This “wedged fit” approach sometimes caused discomfort and put pressure on the urethra. Clinics also struggled to stock the right mix of sizes.
The modern alternative, sold under the brand name Caya, is a single-size diaphragm with a more anatomically contoured shape. It fits about 98% of women (specifically those who would have worn sizes 65 to 80 mm in the old system) and doesn’t require a pelvic exam or professional fitting. It uses a softer spring, has grip dimples to help with insertion, and a finger dome on the rim to make removal easier. The design also avoids pressing on the urethra, which was a common complaint with traditional models.
How to Use a Diaphragm
You can insert a diaphragm up to two hours before sex. Apply a tablespoon or so of spermicide to the inside of the cup and along the rim before inserting it. If more than two hours pass before sex, you’ll need to add more spermicide into your vagina without removing the diaphragm.
After sex, leave the diaphragm in place for at least six hours. This gives the spermicide time to fully neutralize any sperm. You can leave it in for up to 24 hours total, but don’t go longer than that. If you have sex again while the diaphragm is still in, apply another dose of spermicide beforehand.
After removal, wash the diaphragm with mild soap and warm water, let it air dry, and store it in its case. With proper care, a diaphragm typically lasts one to two years before needing replacement.
UTI Risk and Other Side Effects
The most well-documented side effect of diaphragm use is an increased risk of urinary tract infections. A JAMA study found that diaphragm users developed UTIs at roughly three times the rate of women using oral contraceptives: 26.6 infections per 1,000 patient-months compared to 8.9. That increased risk held even after accounting for differences in age, sexual activity, and UTI history. The likely reason is that the rim of the diaphragm can press against the urethra, making it harder for the bladder to empty completely and creating conditions where bacteria thrive.
The modern one-size design reduces this pressure, but if you’re someone who already gets frequent UTIs, a diaphragm may not be your best option. Some people also experience irritation or allergic reactions to spermicide, particularly with frequent use. Symptoms like vaginal burning, itching, or soreness after using spermicide are worth paying attention to, as switching to a different spermicide brand or formulation sometimes helps.
Who Shouldn’t Use a Diaphragm
Certain anatomical conditions make a diaphragm impractical or ineffective. These include uterine prolapse (where the uterus drops lower into the vaginal canal), a large cystocele or rectocele (bulging of the bladder or rectum into the vaginal wall), and a history of recurrent urinary tract infections. If you’ve recently given birth, had pelvic surgery, or experienced significant weight change (roughly 10 pounds or more in either direction), the fit of a traditional diaphragm may need to be reassessed.
Diaphragms do not protect against sexually transmitted infections. They cover the cervix but leave the vaginal walls exposed, so they offer no meaningful barrier against HIV, chlamydia, gonorrhea, or other STIs. If STI protection is a concern, condoms are necessary in addition to the diaphragm.
Advantages and Limitations
The biggest draw of a diaphragm is that it’s hormone-free. For people who experience side effects from hormonal birth control, or who simply prefer not to use hormones, a diaphragm offers a reusable, on-demand option. You only use it when you need it, there’s nothing to remember daily, and it doesn’t affect your menstrual cycle or fertility. Once you stop using it, there’s no waiting period before you can become pregnant.
The trade-off is effectiveness. At 87% with typical use, diaphragms are less reliable than IUDs (over 99%), hormonal implants (over 99%), or even the pill (about 93% with typical use). They also require planning: you need to have the diaphragm and spermicide with you, insert it before sex, and leave it in for hours afterward. For some people, that level of involvement feels manageable. For others, a lower-maintenance method is a better fit.