Birth Control Pills & Antibiotics: Potential Impact on Your Cycle

Question
I hope I don't have to worry.  I was on antibiotics (flagyl) for a week.  Being on the pill I was careful till about a week after the antibiotics finished.  I then had unprotected sex with my husband.  What concerns me is it was at the end of the week of my "sugar pills" and the fact I did not have a very normal period this month.  I had spotting due to either the pap smear that caused me to need the antibiotics or the medication itself.  So I did get my period when I was supposed to but it was on the lighter side.  I hope I didn't lose you....bottom line, should I be worried that I caused a pregnancy?!

Answer
I wouldn't be worried.  Most antibiotics, including flagyl, have a negligible effect on the effectiveness of the pill.  Periods on the pill will sometimes be light, and occasionally don't came at all.  I think pregnancy is very unlikely.  If your next period doesn't come, I'd do a test, but I think it will come on time.