I found one study that addressed diaphragm usage without spermicide vs. using it:
http://humrep.oxfordjournals.org/content/17/4/867.full
The 12-month cumulative pregnancy rate per 100 women for consistent use (defined as no unprotected intercourse and use of only assigned method) for diaphragm-without-spermicide was 19.3 (95% CI, 7.5–31.1) and diaphragm-with-spermicide was 12.3 (95% CI, 2.8–21.8)
http://humrep.oxfordjournals.org/content/17/4/867.full
The 12-month cumulative pregnancy rate per 100 women for consistent use (defined as no unprotected intercourse and use of only assigned method) for diaphragm-without-spermicide was 19.3 (95% CI, 7.5–31.1) and diaphragm-with-spermicide was 12.3 (95% CI, 2.8–21.8)






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