[kildarebabe]

around the 14th of Feb i was put on Antibiotics for a chest infection. i was warned that these would affect my pill. as it was valentines wkend i did have sex. i had a period on the 21st of feb altho it wasnt as long or heavy as it normally is. i have since been feeling sick alot going to the toilet abit more regular and ahould have gotten a period on the 20th of march but didnt. today the 24th i noticed some bleeding but not much. what do you think?

UPDATE: on thursday the 26th i went to doc and she did test which was negative. she advised me to come back to her on monday the 30th and so i did. period was about 10days late on the 30th and test was negative again. she took blood and i am waiting on results. is there still a chance i'm pregnant?

[artlady]

I would say there is always a chance.
Being ill and being on antibiotics can throw off your cycle and even cause you to skip a cycle.
Research indicates that it is a very small percentage of pregnancy's occur because of antibiotic use.

effects of antibiotics on birth control pills may be overstated — except in the case of one antibiotic, rifampin. Studies clearly show that rifampin decreases the effectiveness of birth control pills in preventing ovulation. However, rifampin isn't a widely used antibiotic. Chances are you wouldn't be taking it unless you had tuberculosis or had tested positive for the disease.

Hypothetically speaking, other antibiotics, particularly penicillin and tetracycline derivatives, could impair the effectiveness of birth control pills. However, no large studies have proved such an effect.

Researchers can't rule out the possibility that a small percentage of women may experience decreased effectiveness of birth control pills while taking an antibiotic. And if you're taking a newer, extremely low-dose oral contraceptive, you could be more susceptible to these potential effects from antibiotics.

Birth control pill FAQ: Benefits, risks and choices - MayoClinic.com