Hello and welcome to HCM.
Thank you for your query.
Marvelon, is used as per your doctor's advise is quite effective in preventing
pregnancy, even with unprotected sexual intercourse. Although, it is advised to use condoms, but even without them the efficacy of the
contraceptive pill is about 95 %.
If you have started the regimen as per your doctors instructions and are regular with the pill, then there should be nothing to worry about. Only if you have missed a dose, or have not started the medication at the right time, will the efficacy drop (but only during the first month).
The use of antibiotics along with marvelon, should not affect its efficacy. Do not worry. Chances of you getting pregnant are slim to nothing.
I hope I have succeeded in providing the information you were looking for. Please feel free to write back to me for any further clarifications at: http://doctor.healthcaremagic.com/doctors/dr-shoaib-khan/64581 . I would gladly help you.
Best wishes.