Megadoses of vitamin c will bring on a period... but it has to be regular vitamin c and not the stuff with bioflavanoids or it won't work. There are herbal teas too.
Women used to mega dose on c to induce miscarriage/abortion back in the day.
I'm not sure about the pregnancy question... i was on birth control and got pregnant 4 times on it without missing pills.. so I'm a little untrusting of any method lol