A diaphragm prevents pregnancy by physically blocking sperm from reaching the cervix while spermicidal gel kills any sperm that make contact with it. This two-layer defense, one mechanical and one chemical, makes diaphragms 94% effective with perfect use and about 87% effective with typical use.
How the Two-Layer Defense Works
The diaphragm is a soft, dome-shaped cup made of silicone that you insert into the vagina before sex. It sits snugly over the cervix, which is the narrow opening between the vagina and the uterus. Since the cervix is the only route sperm can take to reach an egg, covering it creates a physical wall that blocks their path.
Before insertion, you apply spermicidal gel to the inside of the dome. This gel contains chemicals that immobilize and kill sperm on contact. So even if some sperm manage to slip around the rim of the diaphragm, the spermicide acts as a second line of defense. Neither layer alone is as reliable as both working together, which is why spermicide is considered essential rather than optional.
Two Types of Diaphragms
There are two diaphragms currently available: the Caya and the Milex. They work the same way but differ in how you get one.
- Caya is a one-size-fits-most diaphragm with a shallower, slightly curved shape and a small inverted dome that makes it easier to insert and remove. Because it comes in a single size, you don’t need a fitting, though a provider can check that it sits properly if you want reassurance.
- Milex is shaped like a small bowl and comes in multiple sizes. Getting one requires a pelvic exam so a provider can measure your anatomy and determine the right fit.
Fit matters because a diaphragm that’s too small can shift out of position during sex, and one that’s too large can feel uncomfortable or fold. Either scenario reduces effectiveness.
How to Use It Correctly
Proper use is the single biggest factor in whether a diaphragm works. The gap between perfect use (94% effective) and typical use (87% effective) comes almost entirely down to human error: forgetting spermicide, inserting it incorrectly, or removing it too early.
To use a diaphragm correctly, you spread about a tablespoon of spermicidal gel inside the dome and around the rim, then fold the diaphragm and slide it into the vagina so it covers the cervix completely. You can insert it up to two hours before sex. After sex, it needs to stay in place for at least six hours so the spermicide has time to neutralize any remaining sperm. If you have sex again before that six-hour window is up, you add more spermicide without removing the diaphragm. Don’t leave it in for more than 24 hours total.
The learning curve is real. Many people need a few practice sessions to feel confident inserting it and checking that it’s seated correctly over the cervix. Running a finger around the rim after insertion to make sure the cervix is fully covered helps confirm placement.
Effectiveness Compared to Other Methods
At 87% typical-use effectiveness, diaphragms prevent pregnancy in roughly 87 out of 100 people using them over a year. That means about 13 out of 100 typical users will become pregnant. For comparison, condoms sit at about 87% with typical use as well, while hormonal IUDs and implants exceed 99%. The diaphragm lands in the middle tier of birth control: meaningfully better than no method, but less forgiving of inconsistent use than long-acting options.
Pairing a diaphragm with condoms significantly improves the odds, and some people choose diaphragms specifically because they want a hormone-free option they control themselves.
Why Diaphragms Can Increase UTI Risk
One well-documented side effect is a higher chance of urinary tract infections. Research has identified two reasons for this. First, the rim of the diaphragm can press against the urethra and partially slow urine flow. In one study, women who felt a sensation of obstruction while wearing a diaphragm showed a significant drop in peak urine flow rate, especially those with a prior UTI history, where flow decreased by an average of 10 milliliters per second. Slower urine flow means bacteria aren’t flushed out as efficiently.
Second, and likely more important, diaphragm use appears to shift the balance of bacteria in the vagina. Current diaphragm users with a UTI history showed heavier growth of the bacteria that cause urinary infections in cultures from the vagina and urethra, and they experienced significantly more infection episodes. Researchers concluded that these changes in vaginal flora may matter more than the physical obstruction itself. If you’re prone to UTIs and considering a diaphragm, this is worth factoring into your decision.
Who It Works Best For
Diaphragms tend to work best for people who are comfortable with insertion, motivated to use the method consistently, and looking for a non-hormonal, reversible option. Because you only use it when you need it, there’s nothing in your body between uses, and fertility returns immediately once you stop. It also won’t affect your menstrual cycle or hormone levels.
It’s a less practical fit if spontaneity is important to you (it requires planning and spermicide each time), if you have difficulty reaching your cervix, or if you’ve recently given birth or had pelvic surgery, since changes in anatomy can affect fit. People with frequent UTIs may also want to weigh that increased risk against the benefits.