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Having unprotected intercourse (without any birth control measures) puts you at risk for pregnancy. The antibiotic use will not affect your menstrual cycles. If your last periods were around Nov 13th, and you have regular periods every 28-30 days, then the fertile period or time of egg release would lie between Nov 21st â Nov 30th. Any unprotected intercourse during this time can result in pregnancy. If you develop other symptoms suggestive of early pregnancy, visit your gynecologist for a thorough examination and blood tests to confirm/rule out pregnancy. Urine pregnancy tests can show false negative results due to less concentration of pregnancy hormone (HCG) in it, and hence cannot be relied upon for accuracy.
Take care.