Condoms, pill, patch, diaphragms? None of these. To prevent pregnancy, 300 years ago, was absolutely natural. They used a little of diluted lemon juice (5:1) to soaked a piece of sponge to fit inside the vagina as tampon, before sex. Well, it was a bit invasive and rough, but historical evidences have shown that once was widely used in the Mediterranean region as a successful contraceptive.
Actually, lemon juice is very acid with a pH between 2.2 and 2.6; scientifically, a concentration of 20% could immobilize 100% of spermatozoa within 30 seconds.

Robert Short, Australian researcher in the Department of Obstetrics and Gynaecology at the University of Melbourne, has recovered our forebears’ birth control system, underlying how this method, extremely cheap, could be useful to curb the birth rate of Third World countries. Contraceptives are too expensive and almost unavailable for much of developing countries.

In the meantime, lemon juice besides preventing pregnancy, in a 10% solution kills very quickly HIV virus. Studies are carrying on, let’s wait and see.

From “ABC News”, 10/10/2002

Important warning:

Obviously, we won’t recommend this method; quite the opposite, we  want you to use condoms in order to prevent pregnancy and sexually transmitted diseases.