Please see the article on the diaphragm for a detailed discussion of the halachic issues.

Fitting of the diaphragm does not render a woman niddah because there is no opening of the cervix. The diaphragm works by blocking the ascent of the sperm into the uterus. It is most effective if used together with a spermicide to kill any sperm that may escape. Its main practical disadvantage is that it has to be put into place prior to each episode of intercourse and left in for 6 hours. Medically, there is no reason that it cannot be used for a long time period.