Any hormonal contraceptive can alter the natural cycle of hormones that constitute a normal menstrual cycle. This is especially true if emergency contraceptive hormones are used. The delicate balance of a woman s normal menstrual cycle is disrupted. The hormones in birth control are five times stronger than the hormones used in estrogen replacement therapy used during menopause symptom treatment. The birth control hormones themselves may cause symptoms that are confused with pregnancy, just because of the strengths used. Cramping, lack of bleeding, breast tenderness, and nausea may all occur with use of these hormones. The best approach in this instance is to take a home pregnancy test approximately two weeks after the episode of intercourse. Most urine pregnancy tests are positive ten days after conception. If not pregnant, menstrual cycles usually regulate to normal within 2-3 cycles after discontinuing hormonal contraception. I hope this helps answer your question and calms your fears. Emergency contraception is very effective and pregnancy is not likely. I am available if you have further questions.