How the Contraceptive Diaphragm Prevents Pregnancy
The diaphragm prevents pregnancy by creating a physical barrier that blocks sperm from entering the cervix and reaching the egg, and when used with spermicide, it provides an additional chemical barrier that immobilizes and kills sperm. 1
Mechanism of Action
The diaphragm works through a dual-barrier approach:
Physical barrier: The diaphragm is a flexible latex or silicone cup that is inserted into the vagina before intercourse and positioned to cover the cervix completely, preventing sperm from passing through the cervical opening. 1, 2
Chemical barrier: When used with spermicide (typically nonoxynol-9), the device provides additional contraceptive protection by killing sperm on contact. 1
Proper positioning is critical: The posterior rim must rest in the posterior fornix, the anterior rim must sit snugly behind the pubic bone, and the cervix should be palpable through the dome of the device to ensure effective sperm blockage. 2
Timing and Duration Requirements
Pre-intercourse insertion: The diaphragm must be inserted into the vagina before sexual intercourse occurs. 1
Post-intercourse retention: The device must remain in place for at least 6 hours after intercourse to ensure all sperm are immobilized or killed. 1
Maximum wear time: The diaphragm should not be left in the vagina for longer than 24 hours to reduce the risk of toxic shock syndrome and other complications. 2
Effectiveness Considerations
Typical-use failure rate: The diaphragm has a typical-use failure rate of 12%, meaning 12 out of 100 women will experience an unintended pregnancy within the first year of use. 1
Perfect-use failure rate: With perfect use, the failure rate drops to 6%, demonstrating that proper technique significantly improves effectiveness. 1
Continuation rate: Only 57% of women continue using the diaphragm at 1 year, which is lower than long-acting reversible contraceptive methods. 1
Important Clinical Caveats
Professional fitting required: Unlike condoms or sponges, diaphragms require fitting by a healthcare professional to ensure proper size and positioning. 1, 2
No STI protection: The diaphragm does not provide adequate protection against sexually transmitted infections, including HIV, despite providing some protection against cervical gonorrhea and chlamydia. 1
Spermicide controversy: While traditionally recommended with spermicide, the evidence for added contraceptive benefit from spermicide use with diaphragms remains inconclusive, though most guidelines still recommend combined use. 3, 4
Urinary tract infection risk: Diaphragm use is associated with increased frequency of urinary tract infections, likely due to pressure on the urethra. 2
Not recommended for adolescents: The American Academy of Pediatrics notes that diaphragms are less commonly recommended for adolescents due to lower effectiveness rates compared to other methods and lack of STI protection. 1