Parity and use-effectiveness with the contraceptive sponge.

The results of a randomized United States study indicated that the Today contraceptive sponge was less effective than the diaphragm (1-year cumulative life-table rate of 17.4 versus 12.9 pregnancies per 100 women, p = 0.01). However, this overall comparison is misleading. Using univariate and multivariate analyses to account for the effects of user characteristics we found parity to be the most important single determinant of effectiveness for users of the sponge, but parity was unimportant as a risk factor for pregnancy among diaphragm users. For nulliparous women the sponge was as effective as a physician-prescribed barrier method (13.9 for sponge, 12.8 for diaphragm, p = 0.45); however, parous women using the sponge were twice as likely to become pregnant (28.3 for sponge, 13.4 for diaphragm, p = 0.001). The effect of parity among sponge users is consistent with the results of international studies of the contraceptive sponge.