: A very simple question : it seems to me that the contraceptive
: pill just prevents the ovule to nest in the vagina and forces it to
: fall every month. But it does not prevent the fertilzation of the 
: ovule. Is it true ? If yes, is there a risk of extra-uterine
: pregnancy, that is the development of the ovule inside the Fallopian
: tube ?

Actually that is not how the pill works, but it *is* how the IUD works.
The oral contraceptive pill actually *prevents* ovulation from occuring
by providing negatve feedback to the pituitary gland, and thus preventing
the LH surge that normally occurs at the time of ovulation.  With the IUD
what happens is that fertilization may occur, but the device prevents
implantation within the wall of the uterus (*not* the vagina).
